varasm: Fix up array_size_for_constructor RAW_DATA_CST handling once again [PR118275]

As the following testcases show (the latter only if I revert the
temporary reversion of the C++ large array speedup), the FEs aren't
really consistent in the type of array CONSTRUCTOR_ELTS indexes,
it can be bitsizetype, but can be sizetype as well.
Given that everything else is able to cope with it just fine,
I'm using build_int_cst which also doesn't care what type it is,
instead of bitsize_int, which I've used in PR117190 fix (previously
it was size_int).

2025-01-03  Jakub Jelinek  <jakub@redhat.com>

	PR c++/118275
	* varasm.cc (array_size_for_constructor): Use build_int_cst
	with TREE_TYPE (index) as first argument, instead of bitsize_int.

	* g++.dg/cpp/embed-18.C: New test.
	* g++.dg/ext/flexary41.C: New test.