Second try: [clang][cmake] Add options to pass in vcs repo and revision info
V2: Fix cmake error when -DLLVM_APPEND_VC_REV=OFF
Clang may be built in an environment where Git is not available. In our case,
Clang is part of a larger monorepo which is not Git-based, and
GenerateVersionFromVCS was not able to get source info.
Provide options to pass in repo and revision info from cmake.
```
cmake \
-DCLANG_VC_REPOSITORY=abc://repo.url.com \
-DCLANG_VC_REVISION=
abcd1234 \
...
```
This would allow us to prepare the source info beforehand and pass it to the
clang binary.
Differential Revision: https://reviews.llvm.org/D148262