[clang] Add getClangVendor() and use it in CodeGenModule.cpp

In 9a38a72f `ProductId` was assigned from the stringified value of
`CLANG_VENDOR`, if that macro was defined. However, `CLANG_VENDOR` is
supposed to be a string, as it is defined (optionally) as such in the
top-level clang `CMakeLists.txt`.

Furthermore, `CLANG_VENDOR` is only passed as a build-time define when
compiling `Version.cpp`, so add a `getClangVendor()` function to
`Version.h`, and use it in `CodegGenModule.cpp`, instead of relying on
the macro.

Fixes: 9a38a72f