ada: Avoid renaming_decl in case of constrained array

ada: Avoid renaming_decl in case of constrained array

This patch avoids rewriting "X: S := F(...);" as "X: S renames F(...);".
That rewrite is incorrect if S is a constrained array subtype,
because it changes the semantics. In the original, the
bounds of X are that of S. But constraints are ignored in
renamings, so the bounds of X would come from F'Result.
This can cause spurious Constraint_Errors in some obscure
cases. It causes unnecessary checks to be inserted, and even
when such checks pass (more common case), they might be less
efficient.

gcc/ada/

	* exp_ch3.adb (Expand_N_Object_Declaration): Avoid transforming to
	a renaming in case of constrained array that comes from source.