c++: Fix up constant expression __builtin_convertvector folding [PR104472]

The following testcase ICEs, because due to the -frounding-math
fold_const_call fails, which is it returns NULL, and returning NULL from
cxx_eval* is wrong, all the callers rely on them to either return folded
value or original with *non_constant_p = true.

The following patch does that, and additionally falls through into the
default case where there is diagnostics for the !ctx->quiet case too.

2022-02-11  Jakub Jelinek  <jakub@redhat.com>

	PR c++/104472
	* constexpr.cc (cxx_eval_internal_function) <case IFN_VEC_CONVERT>:
	Only return fold_const_call result if it is non-NULL.  Otherwise
	fall through into the default: case to return t, set *non_constant_p
	and emit diagnostics if needed.

	* g++.dg/cpp0x/constexpr-104472.C: New test.