[Ada] Consistent wording for missing -gnat2020 switch

[Pierre-Marie de Rodat]

Reuse Error_Msg_Ada_2020_Feature to complain about a missing -gnat2020
switch when code that would be legal in Ada 2020 is compiled in an
earlier language version.

Also, refer to language construct and not to syntactic category of the
missing feature, i.e. "declare expression" and not "declare_expression".

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* par-ch3.adb (P_Identifier_Declarations): Reuse
	Error_Msg_Ada_2020_Feature for object renaming without subtype.
	* par-ch4.adb (P_Primary): Likewise for target name.
	(P_Iterated_Component_Association): Likewise for iterated
	component.
	(P_Declare_Expression): Likewise for declare expression.
	* par-ch6.adb (P_Formal_Part): Likewise for aspect on formal
	parameter.
	* sem_aggr.adb (Resolve_Delta_Aggregate): Ditto.
	* sem_ch8.adb (Analyze_Object_Renaming): Reuse
	Error_Msg_Ada_2020_Feature.
	* sem_ch13.adb (Validate_Aspect_Aggregate): Reuse
	Error_Msg_Ada_2020_Feature; use lower case for "aspect" and
	don't use underscore for "Ada_2020"; don't give up on analysis
	in Ada 2012 mode.
	(Validate_Aspect_Stable_Properties): Reuse
	Error_Msg_Ada_2020_Feature; use lower case for "aspect"; minor
	style fixes.
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