[Patch, objc] maintenance, make Obj{C,C++} LTO-friendly.

[IainS]

This patch is intended to maintain the 'status-quo' in the presence of  
LTO.

In fact, it is a general step forward in that I have identified the  
reason that 'blanket' application of DECL_PRESERVE_P was required and  
got rid of that.

There are places that I intended to round things out a bit more before  
posting - but we are going to have major patch collisions with  
Nathan's work and with resources being so limited, it seems better to  
move forward in smaller steps - rather than spend all the time  
counteracting bit-rot.

It's best viewed as general maintenance - with the side-effect that  
LTO now works - so there's one fewer barriers to enabling that by  
default on Darwin.

This has been tested on:
powerpc*-apple-darwin{8,9} i686-apple-darwin9, x86_64-apple- 
darwin10,  ?86*-unknown-linux-gnu, cris-elf (sim), s390x (compile  
only), mipsisa64-elf(sim), armel-linux-eabi(compile-only).

OK for trunk?
Iain

objc/ChangeLog:
	
	PR objc/44140
	* objc-act.c: build_objc_string_decl() remove declaration.
	(finish_var_decl):  Remove forcing of var output and  
indiscriminate ,marking as "Used".
	(init_objc_symtab): Use integer_zero_node. Use  
short_integer_type_node for the counts.
	(generate_objc_symtab_decl): Remove call to  
forward_declare_categories().  Attatch the
	correct section designation to the symtab.
	(init_module_descriptor): Make the filename "N/A" instead of "".  Use  
integer_zero_node
	instead of building each time. DECL_PRESERVE_P this item.
	(diagnose_missing_method): New.
	(build_next_selector_translation_table): New.
	(build_gnu_selector_translation_table): New.
	(add_objc_string): Incorporate the code from build_objc_string_decl.
	(build_objc_string_decl): Remove.
	(objc_declare_alias): Use integer_zero_node.
	(generate_category): Pass in the current class/category entry.
	Ensure that we finish the variables we originally started, and keep  
them so that they are
	emitted.  (generate_shared_structures): Pass in the current class/ 
category entry. Ensure
	that we finish the variables we originally started so that they are  
output.
	(finish_objc): Reorder code so that we finish variables before  
referencing them.  Save the
	global data before calling meta-daat creation routines, and pass the  
current reference to the
	two main routines. (generate_classref_translation_entry):  
DECL_PRESERVE_P this item
	it is needed by the runtime.  (handle_class_ref): Use finish_var()  
mark items as
	DECL_PRESERVE_P where they are needed by the runtime.
	(handle_impent): Use finish_var() mark items as  DECL_PRESERVE_P  
where they are
	needed by the runtime. (generate_objc_image_info): Use  
integer_zero_node, attatch the
	correct section info.  DECL_PRESERVE_P, this is needed by the runtime.

testsuite/Changelog:

	PR objc/44140

	objc.dg/lto: New.
	objc.dg/lto/trivial-1_0.m: New.
	objc.dg/lto/lto.exp: New.
	obj-c++.dg/lto: New.
	obj-c++.dg/lto/trivial-1_0.mm: New.
	obj-c++.dg/lto/lto.exp: New.

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