fold-const: Handle NON_LVALUE_EXPR in native_encode_initializer [PR114537]

fold-const: Handle NON_LVALUE_EXPR in native_encode_initializer [PR114537]

The following testcase is incorrectly rejected.  The problem is that
for bit-fields native_encode_initializer expects the corresponding
CONSTRUCTOR elt value must be INTEGER_CST, but that isn't the case
here, it is wrapped into NON_LVALUE_EXPR by maybe_wrap_with_location.
We could STRIP_ANY_LOCATION_WRAPPER as well, but as all we are looking for
is INTEGER_CST inside, just looking through NON_LVALUE_EXPR seems easier.

2024-04-04  Jakub Jelinek  <jakub@redhat.com>

	PR c++/114537
	* fold-const.cc (native_encode_initializer): Look through
	NON_LVALUE_EXPR if val is INTEGER_CST.

	* g++.dg/cpp2a/bit-cast16.C: New test.

(cherry picked from commit 1baec8deb014b8a7da58879a407a4c00cdeb5a09)