[clang] [libc] [llvm] [AMDGPU] Use COV6 by default (PR #118515)

[Shilei Tian via cfe-commits]

shiltian wrote:

> Why change the clang default only, and not the backend?

Yeah, initially I thought that was dictated by the front end but forgot we could invoke directly via `opt`, `llc`, etc. Done.

https://github.com/llvm/llvm-project/pull/118515