[Ada] In-place initialization for Initialize_Scalars

[Ada] In-place initialization for Initialize_Scalars

This patch cleans up the implementation of routine Get_Simple_Init_Val. It also
eliminates potentially large and unnecessary tree replications in the context
of object default initialization.

No change in behavior, no test needed.

2018-05-22  Hristian Kirtchev  <kirtchev@adacore.com>

gcc/ada/

	* exp_ch3.adb (Build_Array_Init_Proc): Update the call to
	Needs_Simple_Initialization.
	(Build_Init_Statements): Update the call to Get_Simple_Init_Val.
	(Check_Subtype_Bounds): Renamed to Extract_Subtype_Bounds. Update the
	profile and comment on usage.
	(Default_Initialize_Object): Do not use New_Copy_Tree to set the proper
	Sloc of a value obtained from aspect Default_Value because this could
	potentially replicate large trees. The proper Sloc is now set in
	Get_Simple_Init_Val.
	(Get_Simple_Init_Val): Reorganized by breaking the various cases into
	separate routines. Eliminate the use of global variables.
	(Init_Component): Update the call to Get_Simple_Init_Val.
	(Needs_Simple_Initialization): Update the parameter profile and all
	uses of T.
	(Simple_Init_Defaulted_Type): Copy the value of aspect Default_Value
	and set the proper Sloc.
	* exp_ch3.ads (Get_Simple_Init_Val): Update the parameter profile and
	comment on usage.
	(Needs_Simple_Initialization): Update the parameter profile.

From-SVN: r260526