[debug-early] fix problem with C_TYPE_INCOMPLETE_VARS and TYPE_VFIELD overloading

[Aldy Hernandez <aldyh at redhat dot com>]

In the debug-early work we call dwarf2out early from 
rest_of_decl_compilation.  Dwarf2out, via 
gen_struct_or_union_type_die(), will eventually look at TYPE_VFIELD, 
which is currently being overloaded by the C front-end to keep 
incomplete variables.

Nobody should be looking at the type too in depth if it's incomplete, 
but in this case, the type has just been laid out (layout_decl) so it's 
considered complete, just as we're about to iterate through 
C_TYPE_INCOMPLETE_VARS and fix things up.

To fix my dwarf problem, I've just cached C_TYPE_INCOMPLETE_VARS and 
immediately clear it, as it was going to be cleared after anyhow.

Attached is what I'm committing to the branch, but ideally y'all^Wyall 
front-end folks should use some language specific node.  Nothing was 
obvious in tree-core.h, as most front-end specific things are flags 
(booleans), so I've left this as an exercise to the front-end groupie. 
That being said, if you violently oppose this solution, I'd be more than 
happy to entertain another (hopefully simple) approach.

Aldy

p.s. I wonder how many things are being overloaded by the front-end that 
are being looked at by dwarf2out incorrectly.  Well, nothing that 
triggers a gdb regression....

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