gdb incremental read missing header files

When using DWARF debug information, and incremental (the default)
reading of debug information, GDB will oftentimes miss header files
until the user visits a file that includes, directly or indirectly, the
header file.

Example (the following is a constructed example; the actual problem is
seen when building our embedded kernel that does not include system
header files):

main.c:
-------
extern void print_hello (void);

int
main (int argc, char *argv[])
{
  print_hello ();
  return (0);
}

hello.c:
--------
#include <stdio.h>

void
print_hello (void)
{
  printf ("Hello, world\n");
}

build thusly:
-------------
gcc -c -g3 -O3 main.c
gcc -c -g3 -O3 hello.c
gcc -o missing-headers main.o hello.o

Run gdb:
--------

    gdb missing-headers

At prompt, type:
----------------
    (gdb) info sources
    Source files for which symbols have been read in:

    /emc/dtaylor/bugs/missing-headers/main.c

    Source files for which symbols will be read in on demand:

    /usr/include/bits/stdio2.h, /emc/dtaylor/bugs/missing-headers/hello.c
    (gdb) list features.h:1
    No source file named features.h.
    (gdb) list hello.c:1
    1	#include <stdio.h>
    2	
    3	void
    4	print_hello (void)
    5	{
    6	  printf ("Hello, world\n");
    7	}
    (gdb) info sources
    Source files for which symbols have been read in:

    /emc/dtaylor/bugs/missing-headers/hello.c, /usr/include/stdio.h, 
    /usr/include/features.h, /usr/include/bits/predefs.h, 
    /usr/include/sys/cdefs.h, /usr/include/bits/wordsize.h, 
    /usr/include/gnu/stubs.h, /usr/include/gnu/stubs-64.h, 
    /usr/lib/gcc/x86_64-linux-gnu/4.4/include/stddef.h, /usr/include/bits/types.h, 
    /usr/include/bits/typesizes.h, /usr/include/libio.h, /usr/include/_G_config.h, 
    /usr/include/wchar.h, /usr/lib/gcc/x86_64-linux-gnu/4.4/include/stdarg.h, 
    /usr/include/bits/stdio_lim.h, /usr/include/bits/sys_errlist.h, 
    /usr/include/bits/stdio.h, /usr/include/bits/stdio2.h, 
    /emc/dtaylor/bugs/missing-headers/main.c

    Source files for which symbols will be read in on demand:

    /usr/include/bits/stdio2.h
    (gdb) list features.h:1
    1	/* Copyright (C) 1991-1993,1995-2007,2009,2010 Free Software Foundation, Inc.
    2	   This file is part of the GNU C Library.
    3	
    4	   The GNU C Library is free software; you can redistribute it and/or
    5	   modify it under the terms of the GNU Lesser General Public
    6	   License as published by the Free Software Foundation; either
    7	   version 2.1 of the License, or (at your option) any later version.
    8	
    9	   The GNU C Library is distributed in the hope that it will be useful,
    10	   but WITHOUT ANY WARRANTY; without even the implied warranty of
    (gdb) 

I did this with the 7.9 pre-release (to make sure that there wasn't a
bug fix that I missed), but the same problem occurs in 7.8 and earlier
versions.

By contrast, with STABS most (all?) headers are found without the need
to first 'visit' a file thst includes them.

The above was done on an x86-64 GNU/Linux system, though I doubt that
that matters much to duplicating the problem.