Sema: Constexpr functions must have return statements which have an expr

clang lets programmers be pretty cavalier when it comes to void return
statements in functions which have non-void return types.  However, we
cannot be so forgiving in constexpr functions: evaluation will go off
the rails very quickly.

Instead, keep the return statement in the AST but mark the function as
invalid.  Doing so gives us nice diagnostics while making constexpr
evaluation halt.

This fixes PR21859.