[hardbool] implement OP=, ++ and --, volatile and atomics

[hardbool] implement OP=, ++ and --, volatile and atomics

hardbools didn't behave quite like bools when incremented,
decremented, or otherwise modified using their previous value, as in
+= et al.  Fix that.

Also fix some checking errors that come up when using qualified base
types.

for  gcc/c-family/ChangeLog

	* c-attribs.cc (handle_hardbool_attribute): Create distinct
	enumeration types, with structural equality.  Handle
	base type qualifiers.

for  gcc/c/ChangeLog

	* c-tree.h (C_BOOLEAN_TYPE_P): Cover hardbools as well.
	* c-typeck.cc (convert_lvalue_to_rvalue): New overload and
	wrapper.
	(build_atomic_assign, build_modify_expr): Use it.
	(build_asm_expr, handle_omp-array_sections_1): Simplify with
	it.
	(build_unary_op): Handle hardbools.

for  gcc/testsuite/ChangeLog

	* gcc.dg/torture/hardbool-ai.c: New.
	* gcc.dg/torture/hardbool-vi.c: New.
	* gcc.dg/torture/hardbool.c: Handle NO_BITFIELDS.
	(add1, preinc, postinc, sub1, predec, postdec): New.
	(main): Exercise them.