[Clang] Rename OffloadArch::TargetArch::SPIRV to AMDGCNSPIRV to avoid confusion (#218502)
Following up to
https://github.com/llvm/llvm-project/pull/213362/changes#r3745729994,
the `TargetArch::SPIRV` introduced by #213362 actually corresponds to
`amdgcnspirv`, which is not the same as standard SPIRV: This could prove
rather confusing in the future. Renaming to `TargetArch::AMDGCNSPIRV`
also decouples "`TargetArch::SPIRV`" from AMD-specific SPIRV support, as
SPIRV is used by other vendors as well.