add signed_bool_precision attribute for GIMPLE FE use

add signed_bool_precision attribute for GIMPLE FE use

This adds __attribute__((signed_bool_precision(precision))) to be able
to construct nonstandard boolean types which for the included testcase
is needed to simulate Ada and LTO interaction (Ada uses a 8 bit
precision boolean_type_node).  This will also be useful for vector
unit testcases where we need to produce vector types with
non-standard precision signed boolean type components.

2021-01-06  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/95582
gcc/c-family/
	* c-attribs.c (c_common_attribute_table): Add entry for
	signed_bool_precision.
	(handle_signed_bool_precision_attribute): New.

gcc/testsuite/
	* gcc.dg/pr95582.c: New testcase.