c++: Avoid unnecessary copying in cp_fold [PR94038]

c++: Avoid unnecessary copying in cp_fold [PR94038]

When folding a CALL_EXPR, we can avoid copying it until folding changes
one of its arguments.  And when folding a TREE_VEC, we can avoid using
an intermediate releasing_vec by copying the TREE_VEC as soon as folding
changes one of its arguments, like we do in the CALL_EXPR case.

Incidentally, the CALL_EXPR change also fixes the testcase in PR94038.
The reason is that the call to maybe_constant_value from cp_fold on
the call 'bar<int>()' now reuses the result of the earlier call to
maybe_constant_value from fold_for_warn, via the cv_cache.  This earlier
call passes uid_sensitive=true, whereas the call from cp_fold passes
uid_sensitive=false, and so by reusing the cached result of the earlier
call we now avoid instantiating bar<int> at all.

gcc/cp/ChangeLog:

	PR c++/94038
	* cp-gimplify.c (cp_fold) <case CALL_EXPR>: Move some variable
	declarations closer to their uses.  Copy the CALL_EXPR only
	when one of its arguments has changed.
	<case TREE_VEC>: Instead of first collecting the folded
	arguments into a releasing_vec, just make a copy of the TREE_VEC
	as soon as folding changes one of its arguments.

gcc/testsuite/ChangeLog:

	PR c++/94038
	* g++.dg/warn/pr94038.C: New test.