c++: Fix up cp_build_array_ref COND_EXPR handling [PR120471]

c++: Fix up cp_build_array_ref COND_EXPR handling [PR120471]

The following testcase is miscompiled since the introduction of UBSan,
cp_build_array_ref COND_EXPR handling replaces
(cond ? a : b)[idx] with cond ? a[idx] : b[idx], but if there are
SAVE_EXPRs inside of idx, they will be evaluated just in one of the
branches and the other uses uninitialized temporaries.

Fixed by keeping doing what it did if idx doesn't have side effects
and is invariant.  Otherwise if op1/op2 are ARRAY_TYPE arrays with
invariant addresses or pointers with invariant values, use
SAVE_EXPR <op0>, SAVE_EXPR <idx>, SAVE_EXPR <op0> as a new condition
and SAVE_EXPR <idx> instead of idx for the recursive calls.
Otherwise punt, but if op1/op2 are ARRAY_TYPE, furthermore call
cp_default_conversion on array, so that COND_EXPR with ARRAY_TYPE doesn't
survive in the IL until expansion.

2025-07-01  Jakub Jelinek  <jakub@redhat.com>

	PR c++/120471
gcc/cp/
	* typeck.cc (cp_build_array_ref) <case COND_EXPR>: If idx is not
	INTEGER_CST, don't optimize the case (but cp_default_conversion on
	array early if it has ARRAY_TYPE) or use
	SAVE_EXPR <op0>, SAVE_EXPR <idx>, SAVE_EXPR <op0> as new op0 depending
	on flag_strong_eval_order and whether op1 and op2 are arrays with
	invariant address or tree invariant pointers.  Formatting fixes.
gcc/testsuite/
	* g++.dg/ubsan/pr120471.C: New test.
	* g++.dg/parse/pr120471.C: New test.

(cherry picked from commit 988e87b66882875b14a6cab11c17516863c74a63)