[Ada] Fix missing error on 'Access of constrained array

[Ada] Fix missing error on 'Access of constrained array

For X'Access, the designated subtype of the access type must statically
match the nominal subtype of X.  This patch fixes a bug where the error
was not detected when there is an unrelated declaration of the form "Y :
T := X;", where T is an unconstrained array subtype.

gcc/ada/

	* exp_util.adb (Expand_Subtype_From_Expr): Generate a new
	subtype when Is_Constr_Subt_For_UN_Aliased is True, so the
	Is_Constr_Subt_For_U_Nominal flag will not be set on the
	preexisting subtype.
	* sem_attr.adb, sem_ch3.adb: Minor.