c++: constexpr ctor with RANGE_EXPR index [PR95241]

In the testcase below, the CONSTRUCTOR for 'field' contains a RANGE_EXPR
index:

  {{aggr_init_expr<...>, [1...2]={.off=1}}}

but get_or_insert_ctor_field isn't prepared to handle looking up a
RANGE_EXPR index.

This patch adds limited support to get_or_insert_ctor_field for looking
up a RANGE_EXPR index.  The limited scope of this patch should make it
more suitable for backporting, and more extensive support would be
needed only to handle self-modifying CONSTRUCTORs that contain a
RANGE_EXPR index, but I haven't yet been able to come up with a testcase
that actually creates such a CONSTRUCTOR.

gcc/cp/ChangeLog:

	PR c++/95241
	* constexpr.c (get_or_insert_ctor_field): Add limited support
	for RANGE_EXPR index lookups.

gcc/testsuite/ChangeLog:

	PR c++/95241
	* g++.dg/cpp0x/constexpr-array25.C: New test.