[Ada] Remove obsolete Is_For_Access_Subtype machinery

[Ada] Remove obsolete Is_For_Access_Subtype machinery

This change removes the Is_For_Access_Subtype machinery from the
compiler.  This machinery was devised a long time ago to deal with a
peculiarity of the freezing for access-to-record subtypes but has been
degenerate for quite some time now and does not seem to serve any useful
purpose at this point.

Morever it has an annoying side effect whereby it causes Underlying_Type
to return the (unconstrained) base record type when invoked on the
designated record subtype, which is very problematic for GNATprove.

There should be no functional changes.

2019-07-22  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* einfo.ads (Is_For_Access_Subtype): Delete.
	(Set_Is_For_Access_Subtype): Likewise.
	* einfo.adb (Is_For_Access_Subtype): Likewise.
	(Set_Is_For_Access_Subtype): Likewise.
	(Write_Entity_Flags): Do not write Is_For_Access_Subtype.
	* exp_ch4.adb (Expand_N_Selected_Component): Do not deal with
	it.
	* exp_spark.adb (Expand_SPARK_N_Selected_Component): Likewise.
	* sem_ch4.adb (Analyze_Explicit_Dereference): Likewise.
	* sem_ch3.adb (Build_Discriminated_Subtype): Do not build a
	special private subtype for access-to-record subtypes.

From-SVN: r273682