[clang] [Clang] Track alloca element types to avoid getAllocatedType() calls (PR #181740)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

I'd really like to get some answer about the HLSL code, while we're lookint at it.  CC @llvm-beanz 

Otherwise looks fine to me.

https://github.com/llvm/llvm-project/pull/181740