expr: Fix up constant_byte_string bitfield handling [PR98366]

constant_byte_string now uses a convert_to_bytes function, which doesn't
handle bitfields at all (don't punt on them, just puts them into wrong bits
or bytes).  Furthermore, I don't see a reason why that function should exist
at all, it duplicates native_encode_initializer functionality.
Except that native_encode_initializer punted on flexible array members and 2
tests in the testsuite relied on constant_byte_string handling those.
So, this patch throws away convert_to_bytes, uses native_encode_initializer
instead, but teaches it to handle flexible array members (only in the
non-mask mode with off == -1 for now), furthermore, it adds various corner
case checks that the old implementation was missing (like that STRING_CSTs
use int as length and therefore we shouldn't try to build larger than that
strings, or that native_encode*/native_interpret* APIs require sane
host and target bytes (8-bit on both).

2020-12-19  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/98366
	* fold-const.c (native_encode_initializer): Don't try to
	memset more than total_bytes with off == -1 even if len is large.
	Handle flexible array member initializers if off == -1 and mask is
	NULL.
	* expr.c (convert_to_bytes): Remove.
	(constant_byte_string): Use native_encode_initializer instead of
	convert_to_bytes.  Remove extraneous semicolon.  Punt on various
	corner-cases the APIs don't handle, like sizes > INT_MAX,
	BITS_PER_UNIT != 8, CHAR_BIT != 8.

	* gcc.c-torture/execute/pr98366.c: New test.