ada: Constraint check on tagged build-in-place object decls

ada: Constraint check on tagged build-in-place object decls

In the case of "X : T := F (...);", where T is a constrained
discriminated tagged subtype, perform a constraint check
after F returns. The result of F is allocated by the callee
on the secondary stack in this case.
Note that there are still missing checks for some build-in-place
calls.

gcc/ada/ChangeLog:

	* exp_ch6.adb: Remove a couple of "???" suggesting something that
	we will likely never do.
	(Make_Build_In_Place_Call_In_Object_Declaration):
	When a constraint check is needed, do the check.
	Do it at the call site for now.
	The check is still missing in the untagged case,
	because the caller allocates in that case.
	* sem_ch8.adb (Analyze_Object_Renaming):
	Remove obsolete transformation of a renaming into
	an object declaration. Given that we also (sometimes) tranform
	object declarations into renamings, this transformation was
	adding complexity; the new code in
	Make_Build_In_Place_Call_In_Object_Declaration above
	would need to explicitly avoid the run-time check in the case of
	renamings, because renamings are supposed to ignore the nominal
	subtype. Anyway, it is no longer needed.
	* exp_ch3.adb (Expand_N_Object_Declaration): Rewrite comment;
	it IS clear how to do it, but we haven't done it right yet.