c++: Mark __builtin_convertvector operand as read [PR93557]

c++: Mark __builtin_convertvector operand as read [PR93557]

In C++ we weren't calling mark_exp_read on the __builtin_convertvector first
argument.  I guess it could misbehave even with lambda implicit captures.

Fixed by calling decay_conversion on the argument, we use the argument as
rvalue so we want the standard lvalue to rvalue conversions, but as the
argument must be a vector type, e.g. integral promotions aren't really
needed.

2020-02-05  Jakub Jelinek  <jakub@redhat.com>

	PR c++/93557
	* semantics.c (cp_build_vec_convert): Call decay_conversion on arg
	prior to passing it to c_build_vec_convert.

	* c-c++-common/Wunused-var-17.c: New test.