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

[Erich Keane via cfe-commits]

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

"Meat and Potatoes" patch looks fine to me.  Rest of it is too large to look at beyond scrolling through.  Would be nice to have a few other +1s on this, just because of the size.

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