[Ada] Hang on generic declaration with ambiguous formal package

[Pierre-Marie de Rodat]

This patch fixes the compiler whereby malformed code featuring a generic
package declaration with a formal package may result in an infinite loop
when the name of the formal package is hidden by the formal parameter
itself.

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

gcc/ada/

	* sem_ch12.adb (Analyze_Associations): Add check for errors on
	the generic formal before continuing with instantiation.
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