Implement constant expression support for __real__ and __imag__ on lvalue

complex numbers. Treat complex numbers as arrays of the corresponding component
type, in order to make std::complex behave properly if implemented in terms of
_Complex T.

Apparently libstdc++'s std::complex is implemented this way, and we were
rejecting a member like this:

  constexpr double real() { return __real__ val; }

because it was marked constexpr but unable to produce a constant expression.

llvm-svn: 150895