[clang] [sema] Improve -Wsign-compare (PR #65684)

[via cfe-commits]

apple-fcloutier wrote:

There's two main reasons I didn't go that route. First, we don't have great ways to check that two expressions refer to the same object (that I'm aware of). We can keep track of DeclRefExprs or maybe MemberExprs alone, but that creates a weird cliff of fixit availability. Second, `AnalyzeImplicitConversions` doesn't do recursive descent, so keeping a stack of known facts about inspected values is a good deal more complicated.

If somebody tells me how to do 1, I can probably manage 2 on my own.

https://github.com/llvm/llvm-project/pull/65684