ada: Deconstruct representation clauses required by ASIS

ada: Deconstruct representation clauses required by ASIS

When GNAT was operating in ASIS mode, it was writing internal tables to files,
so we annotated record types for elements stored in these tables with
representation clauses to avoid holes with potentially uninitialized data.

Since ASIS mode has been now deconstructed and we no longer write internal
tables to files, we can remove explicit representation clauses and rely on the
data layout chosen by the compiler.

Code cleanup; behavior is unaffected.

gcc/ada/ChangeLog:

	* lib.ads (Unit_Record): Remove representation clauses and filler
	components
	* lib-load.adb, lib-writ.adb: Remove initialization of data fillers.
	* nlists.adb (Allocate_List_Tables): Remove explicit initialization.
	* repinfo.adb (Exp_Node): Remove representation clauses.
	* sinput.ads (Source_File_Record): Likewise.
	* urealp.adb (Ureal_Entry): Likewise.