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

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Sirraide wrote:

Yeah, it seems the change that ultimately caused this to break was made in 2011, which moved the handling of placeholders for this code path up into `SemaOverload.cpp`, and from what I can tell, the case of either operand of `.*` potentially being an overload set when `.*` is not overloadable was simply overlooked (i.e. the `assert()` is there because this case *was* being handled before, but this commit removed that):
https://github.com/llvm/llvm-project/commit/526ab47a5573422765ac6c55147dfad00f1d703d#diff-3c28567b5e0c77d68f174541a0b77f5a85d093f58b89cd3675ee04a550a44880L7784-L7800


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