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

[Matt Arsenault via cfe-commits]

https://github.com/arsenm commented:

Even as a compiler implementation detail, libclc should not need to consider the address space mapping. 

There is a clang bug if there is different mangling. The itanium mangling should be coming from the source type / original address space, not whatever IR address space value that happens to map to 

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