[clang] [llvm] [WIP] Support the inline asm 'a' constraint on PowerPC (PR #141604)

[Kamau Bridgeman via cfe-commits]

kamaub wrote:

This is a draft PR because the type promotion errors occurring in test case [llvm/test/CodeGen/PowerPC/inline-asm-constraints-error.ll](https://github.com/llvm/llvm-project/pull/141604/files#diff-becfb4d0b9a7b0638d52047d9458c1b8cd9942a318e2895ad0f448b8bdfd2e40) are not being properly handled and need to be resolved and gracefully failed before this patch can go in. Otherwise it would be introducing a way to crash the compiler.

https://github.com/llvm/llvm-project/pull/141604