ada: Get rid of secondary stack for indefinite record types with size clause

ada: Get rid of secondary stack for indefinite record types with size clause

This change eliminates the use of the secondary stack for indefinite record
types for which a valid (object) size clause is specified.  In accordance
with the RM, the compiler accepts (object) size clauses on such types only
if all the components, including those of the variants of the variant part
if any, have a size known at compile time, and only if the clauses specify
a value that is at least as large as the largest possible size of objects
of the types when all the variants are considered.  However, it would still
have used the secondary stack, despite valid (object) size clauses, before
the change, as soon as a variant part was present in the types.

gcc/ada/

	* freeze.ads (Check_Compile_Time_Size): Remove obsolete description
	of usage for the Size_Known_At_Compile_Time flag.
	* freeze.adb (Check_Compile_Time_Size.Size_Known): In the case where
	a variant part is present, do not return False if Esize is known.
	* sem_util.adb (Needs_Secondary_Stack.Caller_Known_Size_Record): Add
	missing "Start of processing" comment.  Return true if either a size
	clause or an object size clause has been given for the first subtype
	of the type.