[Ada] Improve speed of discriminated return types

[Ada] Improve speed of discriminated return types

The compiler now generates faster code for functions that return
discriminated types in many cases where the size is known at compile
time.

2019-08-20  Bob Duff  <duff@adacore.com>

gcc/ada/

	* exp_ch6.adb (Needs_BIP_Alloc_Form): Call
	Requires_Transient_Scope rather than checking constrainedness
	and so forth.  We have previously improved
	Requires_Transient_Scope to return False in various cases,
	notably a limited record with an access discriminant. This
	change takes advantage of that to avoid using the secondary
	stack for functions returning such types.
	(Make_Build_In_Place_Call_In_Allocator): Be consistent by
	calling Needs_BIP_Alloc_Form rather than Is_Constrained and so
	forth.
	* sem_ch4.adb (Analyze_Allocator): The above change causes the
	compiler to generate code that is not legal Ada, in particular
	an uninitialized allocator for indefinite subtype.  This is
	harmless, so we suppress the error message in this case.

From-SVN: r274738