Sema: Diagnose explicitly bound unresolved member expressions decaying into pointers to function type

We would disallow the case where the overloaded member expression is
coming from an address-of operator but we wouldn't issue any diagnostics
when the overloaded member expression comes by way of a function to
pointer decay cast.

Clang's implementation of DR61 is now seemingly complete.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187559 91177308-0d34-0410-b5e6-96231b3b80d8