[Ada] Properly reject unsupported address specifications

In the case of an object declaration with an indefinite nominal subtype
(roughly speaking, that's an object that takes its bounds,
discriminants, and/or tag from its explicit initial value), GNAT does
not support address specifications unless the size of the object is
known at compile time.  In some cases, such unsupported address
specifications were not properly rejected. This could lead to either an
internal error during compilation or (in the class-wide case) to a
warning accompanied by raising Program_Error at run time.

gcc/ada/

	* sem_ch13.adb (Analyze_Attribute_Definition_Clause): Replace
	the existing check for an address specification for an object of
	a class-wide type with a more general check which rejects either
	the class-wide case or the case where the FE would (if the
	address specification were accepted) build a malformed
	tree (specifically, an object renaming declaration with a
	specified address). In the case where the check fails, reject
	the construct at compile time instead of generating an
	unconditional raise of Program_Error.
	* doc/gnat_rm/representation_clauses_and_pragmas.rst: Update
	documentation to reflect these changes.
	* gnat_rm.texi: Regenerate.

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