Ada: Fix misleading diagnostic about abstract new in type derivation

Ada: Fix misleading diagnostic about abstract new in type derivation

The current error message is:

abstract1.ads:7:13: error: "abstract" not allowed here, ignored

but "abstract" is indeed allowed there if you complete the declaration.

The patch changes it to:

abstract1.ads:7:13: error: "abstract" allowed only for record extension, ...

gcc/ada/
	PR ada/55324
	* par-ch3.adb (P_Type_Declaration): Give a better error message
	for illegal "abstract" in a type derivation.

gcc/testsuite/
	* gnat.dg/specs/abstract1.ads: New test.