r196296 - Issue diagnostic when constructor or destructor

[Arthur O'Dwyer]

Richard Smith originally wrote:
> This is a frontend bug, not an IRGen bug; the test case is ill-formed. "return;"
> can be used in a constructor or destructor, but "return <expression-with-type-void>;"
> cannot.

Fariborz Jahanian wrote in response to my comments:
> There is a big difference here. returning a non-void expression in C::C() is allowed
> under an extension warning flag (ext_return_has_expr is ExtWarn<…>) while returning
> a void expression in C::C() is always an error.

Returning a non-void expression in C::C() is NOT allowed, not in any
dialect of C++, as far as I can tell.
The codepath under ext_return_has_expr is actually dealing with a GNU
C (not C++) extension that allows "return void_expr();" (not
"non_void_expr") in a function returning void. Here's how to trigger
that diagnostic:

    $ clang test.c -Wpedantic -c
    test.c:1:26: warning: void function 'bar' should not return void
expression [-Wpedantic]
    void foo(); void bar() { return foo(); }
                             ^      ~~~~~

My point stands, as far as I can tell. And the reason you've had so
much trouble understanding this code is that it's really convoluted
and confusing! We should be trying to *simplify* it, not complicate it
by adding EVEN MORE codepaths and inconsistent behavior.

–Arthur