[clang] [llvm] [clang] Use clang.cfg for Library Defaults (PR #188857)

[Sam Elliott via cfe-commits]

lenary wrote:

I'm not sure how possible this will be for `-fuse-ld=`. There is no argument corresponding to `CLANG_DEFAULT_OBJCOPY` but maybe there should be.

https://github.com/llvm/llvm-project/pull/188857