[Ada] Clean up global variables

[Arnaud Charlet <charlet at adacore dot com>]

This patch eliminates some uses of global variables in the GNAT front end.
No test; no change in behavior.

2016-04-18  Bob Duff  <duff@adacore.com>

	* sem_elab.adb (Output_Calls): Use
	Get_Name_String, to clearly indicate that the global Name_Buffer
	is being used. The previous code used Is_Internal_Name, which
	returns a Boolean, but also has a side effect of setting the
	Name_Buffer. Then it called the other Is_Internal_Name, which uses
	the Name_Buffer for its input. And then it called Error_Msg_N,
	again using the Name_Buffer. We haven't eliminated the global
	usage here, but we've made it a bit clearer.
	This also allows us to have a side-effect-free version of
	Is_Internal_Name.
	* namet.ads, namet.adb: Provide a type Bounded_String, along with
	routines that can be used without using global variables. Provide
	Global_Name_Buffer so existing code can continue to use the
	global. Mark the routines that use globals as obsolete.  New code
	shouldn't call the obsolete ones, and we should clean up existing
	code from time to time.
	Name_Find_Str is renamed as Name_Find.
	* namet.h: Changed as necessary to interface to the new version
	of Namet.
	* bindgen.adb, exp_unst.adb: Name_Find_Str is renamed as
	Name_Find.

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