[libclc] [libclc] Support the generic address space (PR #137183)

[Matt Arsenault via cfe-commits]

arsenm wrote:

> It is actually [supported in the Itanium mangler](https://github.com/llvm/llvm-project/blob/main/clang/lib/AST/ItaniumMangle.cpp#L2786):

I don't remember this part of the hack. There was a recent fix to always use the correct mapping values for AMDGPU when generic address space is enabled (which should be the only mapping, still need to do something about the setAddressSpaceMap hack).

https://github.com/llvm/llvm-project/pull/137183