c++: Handle RANGE_EXPR index in init_subob_ctx [PR97328]

c++: Handle RANGE_EXPR index in init_subob_ctx [PR97328]

In the testcase below, we're ICEing during constexpr evaluation of the
CONSTRUCTOR {.data={{}, [1 ... 7]={}}} of type 'vector'.  The interesting
thing about this CONSTRUCTOR is that it has a RANGE_EXPR index for an
element initializer which doesn't satisfy reduced_constant_expression_p
(because the field 't' is uninitialized).

This is a problem because init_subob_ctx currently punts on setting up a
sub-aggregate initialization context when given a RANGE_EXPR index, so
we later trip over the asserts in verify_ctor_sanity when recursing into
cxx_eval_bare_aggregate on this element initializer.

Fix this by making init_subob_ctx set up an appropriate initialization
context when supplied a RANGE_EXPR index.

gcc/cp/ChangeLog:

	PR c++/97328
	* constexpr.c (init_subob_ctx): Don't punt on RANGE_EXPR
	indexes, instead build a sub-aggregate initialization context
	with no subobject.

gcc/testsuite/ChangeLog:

	PR c++/97328
	* g++.dg/cpp2a/constexpr-init19.C: New test.
	* g++.dg/cpp2a/constexpr-init20.C: New test.

(cherry picked from commit 3d423c6f6a69d87ad52ba3af75f3debd8a8b8810)