c++: -Wunused-value and array init [PR104702]

c++: -Wunused-value and array init [PR104702]

Here, because of problems with the new warning-control code and expressions
that change location, the suppress_warning on the INDIRECT_REF didn't work.
Those problems still need to be worked out, but it's simple to avoid needing
to use suppress_warning in the first place by using a reference instead.

	PR c++/104702

gcc/cp/ChangeLog:

	* init.cc (build_vec_init): Use a reference for the result.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wunused-19.C: New test.