[clang] clang: Remove dest LangAS argument from performAddrSpaceCast (PR #138866)

[John McCall via cfe-commits]

rjmccall wrote:

Merging this was fine because it was just removing the redundant LangAS argument, not removing the abstract handling of address-space conversions in IRGen. I just wanted to note my opposition to the idea of taking that second step, since it had come up in the review.

https://github.com/llvm/llvm-project/pull/138866