[Ada] Compiler rejects legal allocator in record component constraint expression

[Ada] Compiler rejects legal allocator in record component constraint expression

In some cases when a legal allocator which defines a new subtype for the
allocated object occurs as part of a record component constraint
expression, the compiler would incorrectly reject the allocator.

gcc/ada/

	* sem_ch4.adb (Analyze_Allocator): After calling Insert_Action
	to insert a subtype declaration associated with an allocator,
	the subtype declaration will usually be analyzed. But not
	always. Add an explicit call to Preanalyze to cope with the
	unusual case. The subtype declaration must be at least
	preanalyzed before the call to Sem_Ch3.Process_Subtype a little
	while later, during which we analyze an identifier that refers
	to the subtype.