[debug-early] document new world and fix non-dwarf debugging backends

[Aldy Hernandez <aldyh at redhat dot com>]

This patch adjusts the non-dwarf debugging back-ends to work in the new 
world.

Particularly interesting is dbxout, which was generating specific debug 
information for constants not written to memory.  For example, the 
following in C++:

	const int invisible = 0xC0FFEE;

The new infrastructure never sees `invisible' because it doesn't make it 
to the symbol table, especially by the time late_global_decl runs.  What 
I did for this case was to handle it through the early_global_decl() 
hook, which does see `invisible', though it lacks location information. 
 However, since we don't need location information to represent this 
corner case, everything works fine and the pr23205-2.C stabs regression 
is fixed.

I have also updated the documentation for the hooks to reflect reality.

Committed to branch.

Down to 3 sets of individual regressions in the testsuite.  Yay.

Aldy

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