MFC r258054: Update LLDB to upstream r194122 snapshot

Inludes minor changes relative to upstream, for compatibility with
FreeBSD's in-tree LLVM 3.3:

- Reverted LLDB r191806, restoring use of previous API.
- Reverted part of LLDB r189317, restoring previous enum names.
- Work around missing LLVM r192504, using previous registerEHFrames API
(limited functionality).
- Removed PlatformWindows header include and init/terminate calls.

Sponsored by: DARPA, AFRL

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add LLDB bmake infrastructure

This connects LLDB to the build, but it is disabled by default. Add
WITH_LLDB= to src.conf to build it.

Note that LLDB requires a C++11 compiler so is disabled on platforms
using GCC.

Approved by: re (gjb)
Sponsored by: DARPA, AFRL