[Sema] Teach CheckPlaceholderExpr about unaddressable functions.

Given the following C++:

```
void foo();
void foo() __attribute__((enable_if(false, "")));

bool bar() {
  auto P = foo;
  return P == foo;
}
```

We'll currently happily (and correctly) resolve `foo` to the `foo`
overload without `enable_if` when assigning to `P`. However, we'll
complain about an ambiguous overload on the `P == foo` line, because
`Sema::CheckPlaceholderExpr` doesn't recognize that there's only one
`foo` that could possibly work here.

This patch teaches `Sema::CheckPlaceholderExpr` how to properly deal
with such cases.

Grepping for other callers of things like
`Sema::ResolveAndFixSingleFunctionTemplateSpecialization`, it *looks*
like this is the last place that needed to be fixed up. If I'm wrong,
I'll see if there's something we can do that beats what amounts to
whack-a-mole with bugs.

llvm-svn: 272080