Fix a rejects-valid with consteval on overloaded operators

It seems that Clang 11 regressed functionality that was working in
Clang 10 regarding calling a few overloaded operators in an immediate
context. Specifically, we were not checking for immediate invocations
of array subscripting and the arrow operators, but we properly handle
the other overloaded operators.

This fixes the two problematic operators and adds some test coverage to
show they're equivalent to calling the operator directly.

This addresses PR50779.