[Ada] Clean up ??? marks

gcc/ada/

	* binde.adb: No need for ??? marks in Binde, because it is
	superseded by Bindo.
	* bindo-writers.adb (Write_Unit_Closure): Verified that -Ra
	works.
	* exp_ch4.adb, sinfo.ads (Expand_N_Type_Conversion): Rules for
	conversions passed to gigi are documented in sinfo.ads.
	(Expand_N_Unchecked_Type_Conversion): Comment is a duplicate of
	one in sinfo.ads.
	(Expand_N_In): Robert already added sufficient comments years
	after the ??? comment was inserted.
	(Expand_Membership_Minimize_Eliminate_Overflow): I don't see any
	reason why Stand should export Long_Long_Integer'Base -- it
	doesn't export any other base types.
	(Size_In_Storage_Elements): We are doing an allocator, so we
	don't care about sizes in bits.
	(Expand_N_Allocator): PolyORB isn't going to be significantly
	improved, so we're not going to mess with remote access to
	class-wide types.
	(Optimize_Return_Stmt): It's not important to optimize return
	statements in predicate functions -- there are many
	more-important optimizations we could do. Keep part of the
	comment without "???", to clarify why the "and then ...".
	(User_Defined_Primitive_Equality_Op): The optimization doesn't
	seem important enough.
	(Expand_N_Unchecked_Type_Conversion): Refactor to use
	Expand_N_Unchecked_Expression.
	(Make_Array_Comparison_Op): This seems like a case of "it it's
	not broken, don't fix it". Too much risk of causing bugs.
	* debug_a.adb: Remove ??? comments asking why Current_Error_Node
	is maintained unconditionally, and add a comment explaining why.
	* errout.adb: These kinds of minor bugs do indeed exist, but
	we're never going to get around to fixing them "properly", so we
	need this code for robustness.
	* gnatchop.adb (Read_File): Document when read can fail.
	* gnatdll.adb (Parse_Command_Line): Nobody is complaining about
	these arbitrary limits, so no need to use Table. Increase the
	limits just in case.  It is clear from the names what they are
	limits on.
	* gnatlink.adb: Add needed comments.
	(Delete): An existing comment makes clear it's intentional, and
	it's been like that since 1996.
	(Process_Args): Improve comments.
	(Search_Library_Path): Refactoring to avoid deep nesting.
	* inline.adb (Build_Body_To_Inline): Probably won't get around
	to doing that optimization.
	(Is_Unit_Subprogram): No, this should not be moved to Sem_Aux,
	because it is too specialized to this context.
	(Do_Reset): No comment is needed here; it's clear from the
	comment on Reset_Dispatching_Calls. Do_Reset is an artificial
	subprogram; if we had proper iterators, it would just be an if
	statement in the loop.
	(Rewrite_Function_Call): Probably won't get around to doing that
	optimization.
	* layout.adb (Layout_Type): The gigi comment doesn't need to be
	a ??? comment, and it's been that way since 2000.  The
	limitation to scalars will likely never be investigated, and
	it's been that way since 2009.
	* lib.adb (Check_Same_Extended_Unit): This doesn't look like
	something that needs fixing; it looks like a permanent
	workaround.
	* lib-load.adb (Change_Main_Unit_To_Spec): It is good enough in
	practice.
	(Load_Unit): Nobody will ever get around to investigating the
	obscure PMES oddity, and the optimization is not worth the
	trouble.
	* live.adb: It's not worth documenting this. It is used only
	with a debug switch. Nobody who has done significant work on it
	is still around, so it would require substantial investigation.
	* mdll.ads: I see no reason for USE.
	* namet.ads: Routines are obsolete, but they're not going
	anywhere anytime soon (too much work, and surprisingly delicate
	because of dependences on global variables).
	* osint.ads: Minor.
	* osint.adb: Improve comments.
	(Full_Lib_File_Name): Use Smart_Find_File.