[clang] [clang][Driver] Add a new member for CLANG_DEFAULT_LINKER to clang::driver::Driver (NFC) (PR #149784)

[Tarun Prabhu via cfe-commits]

https://github.com/tarunprabhu approved this pull request.

I think I looked at [the other PR](https://github.com/llvm/llvm-project/pull/149786) first. Thanks for splitting this into two separate clang and flang PR's. 

LGTM, but please wait for someone on the clang side to take a look as well.

https://github.com/llvm/llvm-project/pull/149784