[Ada] Make debug printouts more robust

[Ada] Make debug printouts more robust

This patch improves some debug printouts so that they avoid crashing on
invalid data.

In addition, the relevant code uses Global_Name_Buffer all over the
place. This patch cleans up some of those uses, in particular ones in
the same code as the robustness changes, and code called by that code.

gcc/ada/

	* namet.ads, namet.adb (Write_Name_For_Debug): New more-robust
	version of Write_Name.
	(Destroy_Global_Name_Buffer): New procedure to help detect bugs
	related to use of Global_Name_Buffer.  Misc cleanup and comment
	improvements. E.g. we don't need to document every detail of
	debugging printouts, especially since they can change.
	* uname.ads, uname.adb (Write_Unit_Name_For_Debug): New
	more-robust version of Write_Unit_Name.
	(Get_Unit_Name_String): Pass buffer in, instead of using the
	global variable. Misc cleanup. Remove the "special fudge", which
	is apparently not needed, and anyway the comment "the %s or %b
	has already been eliminated" seems wrong.
	(Write_Unit_Name): Call the new version of Get_Unit_Name_String.
	* errout.adb (Set_Msg_Insertion_Unit_Name): Call the new version
	of Get_Unit_Name_String. We pass the global variable here,
	because it's too much trouble to disentangle such uses in
	Errout.
	* sem_util.ads, sem_util.adb, sem_dist.adb
	(Get_Library_Unit_Name): New version of
	Get_Library_Unit_Name_String that avoids usage of the global
	variable.
	* casing.ads, casing.adb, exp_prag.adb, exp_util.adb
	(Set_All_Upper_Case): Remove. There is no need for a wrapper
	here -- code is clearer without it.
	* treepr.adb (Print_Name): Call Write_Name_For_Debug, which
	deals with No_Name (etc), rather than duplicating that here.
	Note that the call to Get_Name_String was superfluous.
	(Tree_Dump): Call Write_Unit_Name_For_Debug instead of
	Write_Unit_Name, which crashes if not Is_Valid_Name.
	* erroutc.ads: Improve comments.
	* erroutc.adb (Set_Msg_Name_Buffer): Call
	Destroy_Global_Name_Buffer to detect potential bugs where it
	incorrectly looks at the global variable.
	* sinput.adb (Write_Location): Call Write_Name_For_Debug instead
	of Write_Name, so it won't blow up on invalid data.
	* sinput.ads: Improve comments; remove some verbosity.
	* libgnat/s-imagef.adb: Fix typo in comment.