fold-const: Don't use build_constructor for non-aggregate types in native_encode_init...

fold-const: Don't use build_constructor for non-aggregate types in native_encode_initializer [PR93121]

The following testcase is rejected, because when trying to encode a zeroing
CONSTRUCTOR, the code was using build_constructor to build initializers for
the elements but when recursing the function handles CONSTRUCTOR only for
aggregate types.

The following patch fixes that by using build_zero_cst instead for
non-aggregates.  Another option would be add handling CONSTRUCTOR for
non-aggregates in native_encode_initializer.  Or we can do both, I guess
the middle-end generally doesn't like CONSTRUCTORs for scalar variables, but
am not 100% sure if the FE doesn't produce those sometimes.

2020-12-04  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/93121
	* fold-const.c (native_encode_initializer): Use build_zero_cst
	instead of build_constructor.

	* g++.dg/cpp2a/bit-cast6.C: New test.