[debug-early] emitting early debug for external variables

[Aldy Hernandez <aldyh at redhat dot com>]

Gentlemen:

Since we pick global symbols to be emitted early from the symbol table, 
we miss out on DECL_EXTERNAL's because they never appear in the symbol 
table:

[rest_of_decl_compilation]:
  else if (TREE_CODE (decl) == VAR_DECL && !DECL_EXTERNAL (decl)
	   && TREE_STATIC (decl))
    varpool_node::get_create (decl);

We could move the early_global_decl call right here, on demand for each 
symbol, since it looks like all front-ends use rest_of_decl_compilation. 
 However, by rest_of_decl_compilation time, we don't know if a symbol 
will be used (the parser hasn't set TREE_USED), and dwarf2out 
specifically tries to avoid generating possibly useless externs:

/* Ignore this VAR_DECL if it refers to a file-scope extern data object
   declaration and if the declaration was never even referenced from
   within this entire compilation unit.  We suppress these DIEs in
   order to save space in the .debug section (by eliminating entries
   which are probably useless).

I know Richi wanted debug information for _everything_, but I doubt 
unused externs would make even him happy.

I can think of various ways of attacking this, none of which are void of 
the words "kludge" or "hack" :).  Did you guys have anything in mind 
here, assuming the present functionality is the end goal here (unused 
externs do NOT appear in the debug info)?

Aldy