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

[ADKaster]

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.

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[ADKaster]

@swift-ci please test

[ADKaster]

Backports #9089 (4c0dc61) from stable/20230725.

Also the same commit is in next as the original upstream d23ef9e

Explanation: Prevents an assertion when trying to do overload resolution on operator .*
Scope: This change only materially affects assertion builds of llvm, such as is the default in pre-release artifacts. The original LLVM PR didn't seem to indicate any risk of changing the meaning of existing code.
Issue: llvm#53815, #8998
Risk: Low risk
Testing: CI, Local testing
Reviewer: @compnerd (for the stable PR 9089)

[ADKaster]

cc @ravikandhadai

[compnerd]

@swift-ci please test

[compnerd]

CC: @DougGregor @airspeedswift

[DougGregor]

This is safe enough to take. Approved for 6.0.