Fix interactions between __builtin_constant_p and constexpr to match

current trunk GCC.

GCC permits information from outside the operand of
__builtin_constant_p (but in the same constant evaluation context) to be
used within that operand; clang now does so too. A few other minor
deviations from GCC's behavior showed up in my testing and are also
fixed (matching GCC):
 * Clang now supports nullptr_t as the argument type for
   __builtin_constant_p
 * Clang now returns true from __builtin_constant_p if called with a
   null pointer
 * Clang now returns true from __builtin_constant_p if called with an
   integer cast to pointer type

llvm-svn: 359059