Add a test for C DR#423 (PR c/65471)

[Joseph Myers]

On Wed, 8 Jun 2016, Marek Polacek wrote:

> Reading <https://gcc.gnu.org/ml/gcc-patches/2016-05/msg01054.html> it occured
> to me that we might resolve c/65471, dealing with type interpretation in
> _Generic, too.  Since it turned out that GCC already does the right thing,
> I'm only adding a new test.  (We should discard qualifiers from controlling 
> expression of _Generic.)
> 
> Regarding the bit-field issue, it seems that we should do nothing and keep
> it implementation-defined for now.
> 
> Tested on x86_64-linux, ok for trunk?

OK.

-- 
Joseph S. Myers
joseph@codesourcery.com