c: Ignore _Atomic on function return type for C2x

c: Ignore _Atomic on function return type for C2x

For C2x it was decided that _Atomic would be completely ignored on
function return types (just as was done for qualifiers in C11 DR#423),
to eliminate the potential for an rvalue returned by a function having
_Atomic-qualified type when an rvalue resulting from lvalue-to-rvalue
conversion could not have such a type.  Implement this for GCC.

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

gcc/c/
	* c-decl.cc (grokdeclarator): Ignore _Atomic on function return
	type for C2x.

gcc/testsuite/
	* gcc.dg/qual-return-9.c, gcc.dg/qual-return-10.c: New tests.