Specialize partial_symtab for DWARF include files

Include files are represented by a partial symtab, but don't expand to
anything.  From dwarf2_psymtab::expand_psymtab:

  if (per_cu == NULL)
    {
      /* It's an include file, no symbols to read for it.
         Everything is in the parent symtab.  */
      readin = true;
      return;
    }

This patch introduces a new specialization of partial_symtab to handle
this case.  In addition to being slightly smaller, I believe an
include file is the only situation where a DWARF psymtab can result in
a null compunit_symtab.  This adds an assert to that effect as well.
This change will simplify one of the psymtab sharing patches.

gdb/ChangeLog
2020-02-26  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (struct dwarf2_include_psymtab): New.
	(dwarf2_create_include_psymtab): Use dwarf2_include_psymtab.
	(dwarf2_psymtab::expand_psymtab, dwarf2_psymtab::readin_p)
	(dwarf2_psymtab::get_compunit_symtab): Remove null checks for
	per_cu_data.