[Sema] Teach overload resolution about unaddressable functions.

Given an expression like `(&Foo)();`, we perform overload resolution as
if we are calling `Foo` directly. This causes problems if `Foo` is a
function that can't have its address taken. This patch teaches overload
resolution to ignore functions that can't have their address taken in
such cases.

Differential Revision: http://reviews.llvm.org/D15590

llvm-svn: 257016