[Ada] Issue errors on wrong context for ghost entities

[Ada] Issue errors on wrong context for ghost entities

References to ghost entities should only occur in ghost context.  This
was not checked systematically on all references.

gcc/ada/

	* sem_ch2.adb (Analyze_Identifier): Add checking for ghost
	context.
	* sem_ch5.adb (Analyze_Implicit_Label_Declaration): Treat
	implicit labels like other entities by setting their ghost
	status according to context.
	* ghost.adb (Check_Ghost_Context): Adapt checking.