[Ada] Implement AI12-0343 Return Statement Checks

[Ada] Implement AI12-0343 Return Statement Checks

2020-06-15  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* einfo.ads (Return_Applies_To): Document special usage for E_Block.
	* einfo.adb (Write_Field8_Name): Write it for E_Block too.
	* exp_ch4.adb (Expand_N_Type_Conversion): Remove implementation of
	the check prescribed by AI05-0073.
	* exp_ch6.adb (Apply_CW_Accessibility_Check): New procedure to apply
	the check prescribed by AI95-344 extracted from...
	(Expand_N_Extended_Return_Statement): Apply the check prescribed by
	AI95-344 to the expression, if present.  Suppress only access checks
	when analyzing the rewritten result.
	(Expand_Simple_Function_Return): ...here.  Rename local variable.
	Call Apply_CW_Accessibility_Check to apply the check prescribed by
	AI95-344, but do not do it for the simple return statement generated
	by the expansion of an extended return statement.  Apply the check
	prescribed by AI05-0073 to all functions returning anonymous access
	type designating a specific tagged type, but not if the expression
	was null or tag checks are suppressed for the type, and use Not In
	operator rather than comparing the tags explicitly.
	* sem.adb (Analyze): Handle all Suppress values.
	* sem_ch6.adb (Analyze_Function_Return): Do not explicitly apply
	predicate checks in the case of an extended return statement.
	Do not apply an implicit conversion to the anonymous access result
	type in the case of the simple return statement generated by the
	expansion of an extended return statement.
	(New_Overloaded_Entity): Small comment tweak.
	* treepr.adb (Print_Node): Fix typo in flag string.