c++: ICE with IMPLICIT_CONV_EXPR in array subscript [PR95508]

c++: ICE with IMPLICIT_CONV_EXPR in array subscript [PR95508]

Since r10-7096 convert_like, when called in a template, creates an
IMPLICIT_CONV_EXPR when we're converting to/from array type.

In this test, we have e[f], and we're converting f (of type class A) to
int, so convert_like in build_new_op_1 created the IMPLICIT_CONV_EXPR
that got into cp_build_array_ref which calls maybe_constant_value.  My
patch above failed to adjust this spot to call fold_non_dependent_expr
instead, which can handle codes like I_C_E in a template.  Fixed by
using a new function maybe_fold_non_dependent_expr, which, if the expr
can't be evaluated to a constant, returns the original expression.

gcc/cp/ChangeLog:

	PR c++/95508
	* constexpr.c (maybe_fold_non_dependent_expr): New.
	* cp-tree.h (maybe_fold_non_dependent_expr): Declare.
	* typeck.c (cp_build_array_ref): Call maybe_fold_non_dependent_expr
	instead of maybe_constant_value.

gcc/testsuite/ChangeLog:

	PR c++/95508
	* g++.dg/template/conv16.C: New test.