[Sema] Make `&function_with_enable_if_attrs` an error

This fixes a bug where one can take the address of a conditionally
enabled function to drop its enable_if guards. For example:

  int foo(int a) __attribute__((enable_if(a > 0, "")));
  int (*p)(int) = &foo;
  int result = p(-1); // compilation succeeds; calls foo(-1)

Overloading logic has been updated to reflect this change, as well.

Functions with enable_if attributes that are always true are still
allowed to have their address taken.

Differential Revision: http://reviews.llvm.org/D13607

llvm-svn: 250090