[clang] [clang-tools-extra] [clang] AST: fix getAs canonicalization of leaf types (PR #155028)

[Younan Zhang via cfe-commits]

https://github.com/zyn0217 approved this pull request.

LGTM, but please wait for other reviewers as the change is really large.

> The user expectation is that getAs only removes top level sugar nodes, leaving all the type sugar on the returned node

So suppose there are really some users who wanted the previous behavior, what should they do? getAs + desugar?

https://github.com/llvm/llvm-project/pull/155028