Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Revert "Ensure .inc files are regenerated when llvm/clang tblgen binaries change"

This reverts commit ab80f0b21fbb9c44d40e6f7a99090188f4ed2f71. The intent
of this change was to avoid possible compilation errors when certain
.inc files were not regenerated, but the method turns out to cause way
more rebuilds than anticipated. Another method will have to be found,
and in the mean time, WITH_CLEAN is the solution that always works.

Fixes: ab80f0b21fbb

Revert "Rework {clang,lldb,llvm}-tblgen invocations to add --write-if-changed"

This reverts commit 8f391d9098083044793ff036dff8c96930fb1553. There are
still a bunch of problems, and apparently ${.ALLSRC} does not work as I
expected.

Fixes: 8f391d909808

Rework {clang,lldb,llvm}-tblgen invocations to add --write-if-changed

This flag ensures that the tblgen tools do not actually touch the
produced .inc file, if there are no changes to the contents. In turn,
this may prevent a number of rebuilds of files that include such .inc
files, saving build time.

While here, ensure that the shell invocations to locate the used tblgen
binary do not show unnecessary error messages.

Reported by: des
MFC after: 1 week

Ensure .inc files are regenerated when llvm/clang tblgen binaries change

When doing a fully incremental build (with WITHOUT_CLEAN enabled), from
a commit before llvm 15 was merged (3264f6b88fce), to a commit after
that, a number of .inc files were not regenerated. This could lead to
unexpected compilation errors when these .inc files were included from
llvm-project sources, similar to:

In file included from /usr/src/contrib/llvm-project/clang/lib/CodeGen/CGBuiltin.cpp:8268:
/usr/obj/usr/src/amd64.amd64/lib/clang/libclang/clang/Basic/arm_mve_builtin_cg.inc:279:18: error: no matching constructor for initialization of 'clang::CodeGen::Address'
Address Val2 = Address(Val1, CharUnits::fromQuantity(2));
^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Work around this by making the .inc files dependent on the tblgen binary
used for generating them. E.g., we can relatively safely assume that if
the binary gets updated, the .inc files must also be updated. (Although
this is not 100% optimal, the gain by complicating things even more is
probaby not worth the effort.)

MFC after: 3 days
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D38770

Move all sources from the llvm project into contrib/llvm-project.

This uses the new layout of the upstream repository, which was recently
migrated to GitHub, and converted into a "monorepo". That is, most of
the earlier separate sub-projects with their own branches and tags were
consolidated into one top-level directory, and are now branched and
tagged together.

Updating the vendor area to match this layout is next.

Add forgotten {llvm,clang}.pre.mk files.