Reinstate r359059, reverted in r359361, with a fix to properly prevent

us emitting the operand of __builtin_constant_p if it has side-effects.

Original commit message:

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: 359367