[clang] [Clang] [Sema] Handle placeholders in '.*' expressions (PR #83103)

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Sirraide wrote:

In my opinion, we ought to get `.*` ought of the way early—as I’m currently doing in this pr—as it makes little sense to do overloading-specific placeholder handling on an operator that isn’t even overloadable—we should instead just handle all placeholders immediately.

https://github.com/llvm/llvm-project/pull/83103