[clang][cmake] Add options to pass in vcs repo and revision info

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