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

[Shilei Tian via llvm-commits]

shiltian wrote:

> @shiltian Could you update MLIR infrastructure for the new default as well? `mlir/lib/Target/LLVM/ROCDL/Target.cpp` and `mlir/lib/Dialect/LLVMIR/IR/ROCDLDialect.cpp`, which both keep an ear on the ABI version, partly for linking in device libraries

https://github.com/llvm/llvm-project/pull/133849

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