[clang] [Sema] Compare canonical conversion function (PR #154158)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

cc @alexfh and @rnk, who should be able to test this.
Testing these changes in advance proved really important in the past as sometimes they tend to break in quite a few tricky and hard-to-catch places on the compiler releases otherwise.

https://github.com/llvm/llvm-project/pull/154158