gimple-fold: Use DECL_PADDING_P in __builtin_clear_padding

gimple-fold: Use DECL_PADDING_P in __builtin_clear_padding

On Wed, Nov 25, 2020 at 12:26:17PM -0500, Jason Merrill wrote:
> I think you want to check DECL_PADDING_P here; the C and C++ front ends set
> it on unnamed bit-fields, and that's what is_empty_type looks at.

While the above has been written in the context of __builtin_bit_cast patch,
I think it applies to __builtin_clear_padding too.

So this patch implements that.  The C FE sets DECL_PADDING_P solely on the
DECL_BIT_FIELD !DECL_NAME FIELD_DECLs, the C++ FE sets it on those and in
another spot I haven't really figured out what it is about.

2020-11-26  Jakub Jelinek  <jakub@redhat.com>

	* gimple-fold.c (clear_padding_union): Ignore DECL_PADDING_P
	fields.
	(clear_padding_type): Ignore DECL_PADDING_P fields, rather than
	DECL_BIT_FIELD with NULL DECL_NAME.