fold-const: Fix up native_encode_initializer missing field handling [PR98193]

fold-const: Fix up native_encode_initializer missing field handling [PR98193]

When native_encode_initializer is called with non-NULL mask (i.e. ATM
bit_cast only), it checks if the current index in the CONSTRUCTOR (if any)
is the next initializable FIELD_DECL, and if not, decrements cnt and
performs the iteration with that FIELD_DECL as field and val of zero
(so that it computes mask properly).  As the testcase shows, I forgot to
set pos to the byte position of the field though (like it is done
for e.g. index referenced FIELD_DECLs in the constructor.

2020-12-09  Jakub Jelinek  <jakub@redhat.com>

	PR c++/98193
	* fold-const.c (native_encode_initializer): Set pos to field's
	byte position if iterating over a field with missing initializer.

	* g++.dg/cpp2a/bit-cast7.C: New test.