Merge branch 'master' into pre-rc8

Update Checked C clang compiler to latest LLVM sources, by merging the
Checked C changes into recent LLVM sources.  The LLVM sources are
approximately at where RC1 for v8.0 was forked.

Testing:
- Passes all Checked C clang tests.
- Passes all but 2 clang tests.  Those are pre-existing failures in the
LLVM sources that we're merging with.