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

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Sirraide wrote:

One more thing: this code seems to not crash and issue a diagnostic just fine if we simply remove the assertion, so that would also be an option, but I didn’t simply want to remove an assertion without fully knowing why it’s there, so I’ve gone with this as an alternative for now (it should be noted that the code path here is shared with `->*`, which of course is overloadable). 

Alternatively, we could also handle placeholders then and there instead of asserting on them. Thinking about this now, I’m not entirely sure what the best solution would be.

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