exp_attr.adb (Expand_N_Attribute_Reference): Add error entry for Library_Level attribute (which should not survive to expansion)

2013-10-14  Robert Dewar  <dewar@adacore.com>

	* exp_attr.adb (Expand_N_Attribute_Reference): Add error
	entry for Library_Level attribute (which should not survive
	to expansion)
	* gnat_rm.texi: Document attribute Library_Level
	* sem_attr.adb (Analyze_Attribute, case Library_Level): Implement
	this new attribute (Set_Boolean_Result): Replaces Set_Result
	(Check_Standard_Prefix): Document that Check_E0 is called
	(Check_System_Prefix): New procedure
	* snames.ads-tmpl: Add entry for Library_Level attribute

2013-10-14  Robert Dewar  <dewar@adacore.com>

	* exp_ch6.adb, sinfo.ads: Minor reformatting.
	* checks.adb (Overlap_Check): Use identifier casing in messages.

From-SVN: r203528