Assembling pending decls before writing their debug info

[Andrew Pinski]

> 
> Hi Guys,
> 
>   There is a problem with unit-at-a-time compilation and DWARF debug
>   info generation.  Consider this small test case which has been
>   derived from GDB's observer.c source file:

There was even more issues with uninitialized variables a month ago.
This was all caused by Mark's patch to fix PR 18556.
This is a regression from 3.4.x.

Thanks,
Andrew Pinski