[clang] [clang][CodeGen] Always use CLANG_VENDOR as a quoted string (PR #75935)

[Shoaib Meenai via cfe-commits]

https://github.com/smeenai requested changes to this pull request.

IIRC, applying definitions like this only to a small set of files was an intentional decision, to minimize rebuilding when their values changed. It matters much more for the commit hash than the vendor, but passing defines like this to every single compilation in Clang is ugly IMO. Can we instead add a function to Version.cpp to access the vendor (so that the correct type gets encoded in the function signature) and use that from CodeGenModule.cpp instead of referencing `CLANG_VENDOR` directly?

https://github.com/llvm/llvm-project/pull/75935