[Ada] tech debt: Clean up Uint fields, such as Esize

[Ada] tech debt: Clean up Uint fields, such as Esize

gcc/ada/

	* atree.ads: Comment improvements. How is a "completely new
	node" different from a "new node"? Document default values
	corresponding to field types.
	* exp_ch7.adb (Process_Tagged_Type_Declaration): Use
	higher-level Scope_Depth instead of Scope_Depth_Value.  Remove
	confusing comment: not clear what a "true" library level package
	is.
	* uintp.adb (Image_Out): Print No_Uint in a more readable way.
	* gen_il-gen.adb, gen_il-gen-gen_entities.adb,
	gen_il-gen-gen_nodes.adb, gen_il-types.ads: Tighten up the
	subtypes of fields whose type is Uint, where possible; use
	more-constrained subtypes such as Unat.
	* einfo-utils.adb, einfo-utils.ads, exp_attr.adb,
	exp_ch3.adb, exp_intr.adb, exp_unst.adb, exp_util.adb,
	freeze.adb, repinfo.adb, sem.adb, sem_ch12.adb, sem_ch13.adb,
	sem_ch3.adb, sem_ch8.adb, sem_util.adb, sprint.adb, treepr.adb:
	No longer use Uint_0 to indicate "unknown" or "not yet known"
	for various fields whose type is Uint. Use No_Uint for that,
	except in a small number of legacy cases that cause test
	failures. Protect many queries of such fields with calls to
	Known_... functions. Improve comments.
	* exp_aggr.adb: Likewise.
	(Is_OK_Aggregate): Check whether Csiz is present.
	(Aggr_Assignment_OK_For_Backend): Ensure we do not access an
	uninitialized size.
	* exp_strm.adb (Build_Elementary_Input_Call,
	Build_Elementary_Write_Call): Check whether P_Size is present.
	* cstand.adb: Leave Component_Size of Any_Composite unknown.
	Similar for RM_Size of Standard_Exception_Type.  These should
	not be used.
	* einfo.ads: Comment improvements.
	* exp_disp.ads: Minor.
	* gen_il-internals.ads, gen_il-internals.adb: Minor.
	* sinfo-utils.adb: Take advantage of full-coverage rules.
	* types.h: Minor.