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

[Petr Hosek via cfe-commits]

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

I agree with @smeenai, we shouldn't be setting `CLANG_VENDOR` as a global define.

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