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

[Andrey Ali Khan Bolshakov via cfe-commits]

https://github.com/bolshakov-a commented:

Looks good, thanks! I didn't review the whole PR, just noticed a couple of issues.

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