ada: Split Library_Unit using multiple wrappers

ada: Split Library_Unit using multiple wrappers

The Library_Unit field was used for all sorts of different purposes,
which led to confusing code.

This patch splits Library_Unit into much more specific wrapper
subprograms that should be called instead of [Set_]Library_Unit.
Predicates and pragmas Assert are used to catch misuses of these.
We document the semantics, especially "surprising" cases (e.g.
internally-generated with clauses can refer to package bodies).

This change does not fix gigi, codepeer, spark, or llvm
to use the new wrappers; so far, they are used only in
the GNAT front end.

gcc/ada/ChangeLog:

	* sinfo.ads (Library_Unit): Rewrite documentation. Note that
	the "??? not (always) true..." comment was not true;
	the Subunit_Parent never points to the spec.
	(N_Compilation_Unit): Improve documentation. The Aux_ node
	was not created to solve the mentioned problems; it was
	created because the size of nodes was limited.
	Misc doc improvements.
	* sinfo-utils.ads: Add new wrappers for Library_Unit field.
	Use subtypes with predicates for the parameters.
	(First_Real_Statement): Still used in codepeer.
	* sinfo-utils.adb: Add new wrappers for Library_Unit field,
	with suitable assertions.
	* sem_prag.adb: Use new field wrapper names.
	(Matching_Name): New name for Same_Name to avoid
	potential confusion with the other function with the
	same name (Sem_Util.Same_Name), which is also called
	in this same file.
	(Matching_Convention): Change Same_Convention to match
	Matching_Name.
	* sem_util.ads (Same_Name): Improve comments; the old comment
	implied that it works for all names, which was not true.
	* sem_util.adb: Use new field wrapper names.
	* gen_il-gen.adb: Rename N_Unit_Body to be N_Lib_Unit_Body.
	Plain "unit" is ambiguous in Ada (library unit, compilation
	unit, program unit, etc).
	Add new union types N_Lib_Unit_Declaration and
	N_Lib_Unit_Renaming_Declaration.
	* gen_il-gen-gen_nodes.adb (Compute_Ranges): Raise exception
	earlier (it is already raised later, in Verify_Type_Table).
	Add a comment explaining why it might be raised.
	* gen_il-types.ads: Rename N_Unit_Body to be N_Lib_Unit_Body, and add
	new N_Lib_Unit_Declaration and N_Lib_Unit_Renaming_Declaration.
	* einfo.ads: Fix obsolete comment (was left over from before
	the "variable-sized nodes").
	* exp_ch7.adb: Use new field wrapper names.
	* exp_disp.adb: Use new field wrapper names.
	* exp_unst.adb: Use new field wrapper names.
	* exp_util.adb: Use new field wrapper names.
	* fe.h: Add new field wrapper names. These are currently not
	used in gigi, but this change prepares for using them in
	gigi.
	* inline.adb: Use new field wrapper names.
	* lib.adb: Use new field wrapper names.
	Comment improvements.
	* lib-load.adb: Use new field wrapper names.
	Minor cleanup.
	* lib-writ.adb: Use new field wrapper names.
	* live.adb: Use new field wrapper names.
	* par-load.adb: Use new field wrapper names.
	Comment improvements. Minor cleanup.
	* rtsfind.adb: Use new field wrapper names.
	* sem.adb: Use new field wrapper names.
	* sem_ch10.adb: Use new field wrapper names.
	Comment improvements. Minor cleanup.
	* sem_ch12.adb: Use new field wrapper names.
	* sem_ch7.adb: Use new field wrapper names.
	* sem_ch8.adb: Use new field wrapper names.
	* sem_elab.adb: Use new field wrapper names.
	Comment improvements.
	* errout.adb (Output_Source_Line): Fix blowup in some
	obscure cases, where List_Pragmas is not fully set up.