ada: Inspect deferred constant completions in missing contexts

ada: Inspect deferred constant completions in missing contexts

Namely declare expressions and statement lists, which can have object
declarations in -gnatX mode.

gcc/ada/ChangeLog:

	* sem_util.ads: Introduce Inspect_Deferred_Constant_Completion
	on a single object declaration, to better factorize the code
	* sem_util.adb: Introduce aforementioned overload
	* sem_ch4.adb (Analyze_Expression_With_Actions): Check deferred
	constant completions
	* sem_ch5.adb (Analyze_Statements): Check deferred constant
	completions