arcanist: use FreeBSD/git project repository instead of FreeBSD/svn

Current `.arcconfig` specifies explicit mapping to the FreeBSD/SVN
project in Phabricator, which is inactive. This mapping only gets
updated to the current "production" (FreeBSD/git) project when the
underlying change is committed.
Update arcanist configuration to create all new diffs in the
FreeBSD/git project.

Reviewed By: dim, emaste
Differential Revision: https://reviews.freebsd.org/D36204

Remove history.immutable from .arcconfig

The `history.immutable` setting prevents arcanist from updating
the commit messages with the Differential URL and therefore
makes updating patches awkward with a rebase workflow.

In case this new behaviour is not wanted the old one can be restored
by running `arc set-config --local history.immutable true`.

Test Plan: `arc diff --create HEAD^` adds the metadata now.

Reviewed By: #phabric-admin, imp, lwhsu
Differential Revision: https://reviews.freebsd.org/D27971

Adjust .arcconfig to land onto 'main' by default

Otherwise, commands like "arc land" will default to 'master' instead.

MFC after: immediately

arcconfig: add callsign again

Problem

When using git-svn or other non-pure-svn tooling the original subversion
URL is not present. This causes arcanist/phabricator to be unable to
determine which repository is being modified.

Solution

Restore callsign to .arcconfig to enable exact repository matching even
with git-svn.

Reported By: jhb

Remove upstream files and directories from vendor/lldb/dist that we do
not use. This saves on repository space, and reduces the number of tree
conflicts when merging.

Remove upstream files and directories from vendor/libc++/dist that we do
not use. This saves on repository space, and reduces the number of tree
conflicts when merging.

Remove upstream files and directories from vendor/compiler-rt/dist that
we do not use. This saves on repository space, and reduces the number
of tree conflicts when merging.

Remove upstream files and directories from vendor/clang/dist that we do
not use. This saves on repository space, and reduces the number of tree
conflicts when merging.

Remove upstream files and directories from vendor/llvm/dist that we do
not use. This saves on repository space, and reduces the number of tree
conflicts when merging.

Vendor import of llvm trunk r338150:
https://llvm.org/svn/llvm-project/llvm/trunk@338150

Vendor import of lld trunk r321017:
https://llvm.org/svn/llvm-project/lld/trunk@321017

Vendor import of libc++ trunk r321017:
https://llvm.org/svn/llvm-project/libcxx/trunk@321017

Vendor import of compiler-rt trunk r321017:
https://llvm.org/svn/llvm-project/compiler-rt/trunk@321017

Vendor import of clang trunk r321017:
https://llvm.org/svn/llvm-project/cfe/trunk@321017

Vendor import of llvm trunk r290819:
https://llvm.org/svn/llvm-project/llvm/trunk@290819

callsign isn't required anymore

Vendor import of lldb release_39 branch r276489:
https://llvm.org/svn/llvm-project/lldb/branches/release_39@276489

Vendor import of lld release_39 branch r276489:
https://llvm.org/svn/llvm-project/lld/branches/release_39@276489

Vendor import of libc++ release_39 branch r276489:
https://llvm.org/svn/llvm-project/libcxx/branches/release_39@276489

Vendor import of compiler-rt release_39 branch r276489:
https://llvm.org/svn/llvm-project/compiler-rt/branches/release_39@276489

Vendor import of clang release_39 branch r276489:
https://llvm.org/svn/llvm-project/cfe/branches/release_39@276489

Vendor import of llvm release_39 branch r276489:
https://llvm.org/svn/llvm-project/llvm/branches/release_39@276489

Remove project.name which is a product of a bygone era.

Vendor import of lldb trunk r256945:
https://llvm.org/svn/llvm-project/lldb/trunk@256945

Import libc++ 3.7.0 release (r246257).

Add repository.callsign, to help arcanist figure out what repo it's
doing things on when not using Subversion.

With hat: phabric-admin@
Sponsored by: Absolight

Phabricator: enable "history.immutable":
With certain arc workflows leaving history.immutable as false
results in using the upstream template instead of our usual
commit template. Since the git workflow issues alluded to in my
prior commit message can be worked around, set history.immutable
once again.

phabricator related changes:
- don't lint either contrib or crypto: these are both externally written
directories
- add additional linters for spelling (check common typos like teh ->
the)
- chmod linter checks for executible bit on bad files
- merge-conflict checks for merge conflict tokens then may have been
resolved incorrectly
- filename checks for back characters in filenames
- json for json syntax correctness

- remove history.immutable: it is meaningless on subversion, and causes
workflow problems when trying to use git. It it set to 'true' by
default with hg

Vendor import of lld trunk r233088:
https://llvm.org/svn/llvm-project/lld/trunk@233088

Import compiler-rt trunk r224034.

https://llvm.org/svn/llvm-project/compiler-rt/trunk@224034

Vendor import of clang RELEASE_350/final tag r216957 (effectively, 3.5.0 release):
https://llvm.org/svn/llvm-project/cfe/tags/RELEASE_350/final@216957

Vendor import of llvm RELEASE_350/final tag r216957 (effectively, 3.5.0 release):
https://llvm.org/svn/llvm-project/llvm/tags/RELEASE_350/final@216957

Update the URL to the phabricator instance.

Sponsored by: The FreeBSD Foundation

Prevent arc commands from overwriting history

Add the missing coma

Add project name to the arc config it is still required when arcanist is used with SVN

arcconfig: add one

Add a .arcconfig to allow arc to work in its usual way.

Vendor import of clang trunk r178860:
http://llvm.org/svn/llvm-project/cfe/trunk@178860

Vendor import of llvm trunk r178860:
http://llvm.org/svn/llvm-project/llvm/trunk@178860

Import compiler-rt r172839.