Disallow the operand of __builtin_constant_p from modifying enclosing

Disallow the operand of __builtin_constant_p from modifying enclosing
state when it's encountered while evaluating a constexpr function.

We attempt to follow GCC trunk's behavior here, but it is somewhat
inscrutible, so our behavior is only approximately the same for now.
Specifically, we only permit modification of objects whose lifetime
began within the operand of the __builtin_constant_p. GCC appears to
have effectively the same restriction, but also some unknown restriction
based on where and how the local state of the constexpr function is
mentioned within the operand (see added testcases).

llvm-svn: 359958