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

[Pierre van Houtryve via cfe-commits]

Pierre-vh wrote:

@shiltian Please also remove `warn_drv_amdgpu_cov6` - it's a warning I added while v6 was being worked on.
Now that it's supported by the stack we can remove it.

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