[clang] [libc] [llvm] Reapply "[AMDGPU] Use COV6 by default (#118515)" (PR #130963)

[Shilei Tian via cfe-commits]

shiltian wrote:

CC @jdoerfert @ye-luo Once this is merged, ROCm 6.3 will be needed to run any program compiled for AMDGPU.

https://github.com/llvm/llvm-project/pull/130963