[clang] [clang][NFC] Replace TypeAlignment with alignof(T) (PR #69185)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> I'm not sure we care (the api is not meant to be stable - that's why we have a c wrapper) but @AaronBallman should weight on that

Basically none of our internal C++ APIs have a stability guarantee, so we're free to refactor as we see fit. In this case, I don't see much danger to removing `TypeAlignment` if it's no longer being used.

https://github.com/llvm/llvm-project/pull/69185