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

When analysing whether we should handle a binary expression as an
overloaded operator call or a builtin operator, we were calling
`checkPlaceholderForOverload()`, which takes care of any placeholders
that are not overload sets—which would usually make sense since those
need to be handled as part of overload resolution.

Unfortunately, we were also doing that for `.*`, which is not
overloadable, and then proceeding to create a builtin operator anyway,
which would crash if the RHS happened to be an unresolved overload set
(due hitting an assertion in `CreateBuiltinBinOp()`—specifically, in one
of its callees—in the `.*` case that makes sure its arguments aren’t
placeholders).

This pr instead makes it so we check for *all* placeholders early if the
operator is `.*`.

It’s worth noting that,
1. In the `.*` case, we now additionally also check for *any*
placeholders (not just non-overload-sets) in the LHS; this shouldn’t
make a difference, however—at least I couldn’t think of a way to trigger
the assertion with an overload set as the LHS of `.*`; it is worth
noting that the assertion in question would also complain if the LHS
happened to be of placeholder type, though.
2. There is another case in which we also don’t perform overload
resolution—namely `=` if the LHS is not of class or enumeration type
after handling non-overload-set placeholders—as in the `.*` case, but
similarly to 1., I first couldn’t think of a way of getting this case to
crash, and secondly, `CreateBuiltinBinOp()` doesn’t seem to care about
placeholders in the LHS or RHS in the `=` case (from what I can tell,
it, or rather one of its callees, only checks that the LHS is not a
pseudo-object type, but those will have already been handled by the call
to `checkPlaceholderForOverload()` by the time we get to this function),
so I don’t think this case suffers from the same problem.

This fixes llvm#53815.

---------

Co-authored-by: Aaron Ballman <[email protected]>

clang/docs/ReleaseNotes.rst

@@ -423,6 +423,8 @@ Bug Fixes to C++ Support
   (`#82258 <https://github.com/llvm/llvm-project/issues/82258>`_)
 - Correctly immediate-escalate lambda conversion functions.
   (`#82258 <https://github.com/llvm/llvm-project/issues/82258>`_)
+- Fix a crash when an unresolved overload set is encountered on the RHS of a ``.*`` operator.
+  (`#53815 <https://github.com/llvm/llvm-project/issues/53815>`_)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^