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

[Oliver Hunt via cfe-commits]

https://github.com/ojhunt requested changes to this pull request.

This seems like it has the potential to be a significant memory regression, so I've poked people who may know the answer, or know who would.

That said this also needs tests :D

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