[Ada] Parser and lexer cleanup

[Ada] Parser and lexer cleanup

This patch makes various minor cleanup changes to the parser.
No change in behavior.

gcc/ada/

	* par-tchk.adb, par-util.adb, prep.adb, prepcomp.adb, scng.adb:
	Use "in" instead of chains of "=" connected with "or else".
	Likewise for "not in", "/=", "and then". Misc cleanup.
	* par-ch10.adb, par-ch12.adb, par-ch13.adb, par-ch4.adb: Likewise.
	* par-ch8.adb, par-ch9.adb, par-endh.adb, par-sync.adb: Likewise.
	* par.adb
	(Pf_Rec): Remove filler, which was added August 25, 1993 to get
	around a compiler limitation that no longer exists. Minor cleanup.
	Remove useless qualfications.
	* par-ch3.adb: Remove redundant return statements.
	(Component_Scan_Loop): Remove loop name; there are no nested
	loops, so it's unnecessary and possibly misleading, and it causes
	too-long lines.
	* par-ch5.adb: DRY: Remove comments that repeat the comments in
	par.adb.
	(P_Sequence_Of_Statements): It is better to initialize things on
	the declaration. And constants are better than variables.
	(Test_Statement_Required): Remove unnecessary insertion of a null
	statement.
	* par-ch6.adb, par-ch7.adb: DRY: Remove comments that repeat the
	comments in par.adb.