[Ada] Cleanup expansion of attribute Constrained

[Ada] Cleanup expansion of attribute Constrained

gcc/ada/

	* exp_attr.adb (Expand_N_Attribute_Reference): Replace calls to
	Sloc with a local constant Loc; remove call to
	Analyze_And_Resolve and return, which is exactly what happens
	anyway (and other branches in the Constrained declare block
	appear to rely on analysis, resolution and returning happening
	in all cases).
	* sem_util.adb: Remove useless parens.