[Ada] Reject junk syntax for Contract_Cases/Test_Case/Subprogram_Variant

[Pierre-Marie de Rodat]

Reject contracts Contract_Cases, Test_Case and Subprogram_Variant whose
expression is either "null", "(null record)" or has extra parentheses.

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

gcc/ada/

	* sem_ch13.adb (Analyze_Aspect_Specifications): Add a codefix
	for extra parentheses around aspect Annotate expression; reject
	"(null record)" aggregate and extra parentheses around aspect
	Test_Case expression.
	* sem_prag.adb (Analyze_Pragma): Reject "null", "(null record)"
	and extra parentheses around pragma Contract_Cases; likewise for
	pragma Subprogram_Variant.
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