c++: fix array cleanup with throwing temp dtor

While working on PR66139 I noticed that if the destructor of a temporary
created during array initialization throws, we were failing to destroy the
last array element constructed.  Throwing destructors are rare since C++11,
but this should be fixed.

gcc/cp/ChangeLog:

	* init.c (build_vec_init): Append the decrement to elt_init.

gcc/testsuite/ChangeLog:

	* g++.dg/eh/array2.C: New test.