fix debug/23190

[Richard Henderson <rth at redhat dot com>]

For 4.0, the fix appears to be as simple as is described in the pr log: 
split the debug emission out and invoke it after cgraph_optimize.  Doing
that means that the variable gets emitted if it ever will be, so that
its DECL_RTL gets filled in.

For 4.1, cgraph gets a bit more clever, which makes the problem harder.
This problem won't be solved correctly until cgraph manages *everything*
with respect to the object symbol table.

I found that there was no good way for cgraph to actually decide after
the fact that it could avoid emitting a particular variable.  Once the
DECL_RTL for a variable has been set, you don't know who has it and 
what they've done with it.  So I've partially (or perhaps totally) 
disabled cgraph_varpool_remove_unreferenced_decls.

I found that we weren't emitting debug info for local statics *at all*
without optimization.  That had gotten moved to
cgraph_varpool_assemble_pending_decls, but we only went down that path
with -funit-at-a-time.  Thus the change to rest_of_decl_compilation.

Tested on i686, ia64, and x86_64 linux.


r~

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