[gdb] Fix missing symtab includes

[gdb] Fix missing symtab includes

[ The test-case requires commit c1a66c0629 "[gdb] Expand symbolless symtabs
using maint expand-symtabs". ]

Consider the debug info for the test-case included in this patch.  It consists
of a PU:
...
 <0><d2>: Abbrev Number: 2 (DW_TAG_partial_unit)
 <1><d3>: Abbrev Number: 0
...
imported by a CU:
...
 <0><df>: Abbrev Number: 2 (DW_TAG_compile_unit)
    <e0>   DW_AT_language    : 2        (non-ANSI C)
    <e1>   DW_AT_stmt_list   : 0xe9
 <1><e5>: Abbrev Number: 3 (DW_TAG_imported_unit)
    <e6>   DW_AT_import      : <0xd2>   [Abbrev Number: 2]
 <1><ea>: Abbrev Number: 0
...
and the CU has a dw2-symtab-includes.h file in the .debug_line file name
table:
...
 The Directory Table (offset 0x101):
  1     /data/gdb_versions/devel/src/gdb/testsuite/gdb.dwarf2

 The File Name Table (offset 0x138):
  Entry Dir     Time    Size    Name
  1     1       0       0       dw2-symtab-includes.h
...

After expanding all symtabs, we can see the CU listed in the user field of the
PU, and vice-versa the PU listed in the includes of the CU:
...
$ gdb.sh -batch \
  -iex "set language c" \
  outputs/gdb.dwarf2/dw2-symtab-includes/dw2-symtab-includes \
  -ex "maint expand-symtabs" \
  -ex "maint info symtabs"
  ...
  { ((struct compunit_symtab *) 0x394dd60)
    debugformat DWARF 2
    producer (null)
    dirname (null)
    blockvector ((struct blockvector *) 0x394dea0)
    user ((struct compunit_symtab *) 0x394dba0)
  }
  { ((struct compunit_symtab *) 0x394dba0)
    debugformat DWARF 2
    producer (null)
    dirname (null)
    blockvector ((struct blockvector *) 0x394dd10)
    user ((struct compunit_symtab *) (null))
    ( includes
      ((struct compunit_symtab *) 0x394dd60)
    )
  }
...

But if we instead only expand the symtab for the dw2-symtab-includes.h file,
the includes and user links are gone:
...
$ gdb -batch \
  -iex "set language c" \
  outputs/gdb.dwarf2/dw2-symtab-includes/dw2-symtab-includes \
  -ex "maint expand-symtabs dw2-symtab-includes.h" \
  -ex "maint info symtabs"
  ...
  { ((struct compunit_symtab *) 0x2728210)
    debugformat DWARF 2
    producer (null)
    dirname (null)
    blockvector ((struct blockvector *) 0x2728350)
    user ((struct compunit_symtab *) (null))
  }
  { ((struct compunit_symtab *) 0x2728050)
    debugformat DWARF 2
    producer (null)
    dirname (null)
    blockvector ((struct blockvector *) 0x27281c0)
    user ((struct compunit_symtab *) (null))
  }
...

The includes are calculated by process_cu_includes in gdb/dwarf2/read.c.

In the case of expanding all symtabs:
- the CU partial symtab is expanded using psymtab_to_symtab
- psymtab_to_symtab calls dwarf2_psymtab::read_symtab
- dwarf2_psymtab::read_symtab calls dwarf2_psymtab::expand_psymtab
- dwarf2_psymtab::read_symtab calls process_cu_includes, and we have the
  includes

In the case of expanding the symtab for dw2-symtab-includes.h:
- the dw2-symtab-includes.h partial symtab is expanded using psymtab_to_symtab
- psymtab_to_symtab calls dwarf2_include_psymtab::read_symtab
- dwarf2_include_psymtab::read_symtab calls
  dwarf2_include_psymtab::expand_psymtab
- dwarf2_include_psymtab::expand_psymtab calls
  partial_symtab::expand_dependencies
- partial_symtab::expand_dependencies calls dwarf2_psymtab::expand_psymtab
  for the CU partial symtab
- the CU partial symtab is expanded using dwarf2_psymtab::expand_psymtab
- process_cu_includes is never called

Fix this by making sure in dwarf2_include_psymtab::read_symtab that
read_symtab is called for the CU partial symtab.

Tested on x86_64-linux, with native, and target board cc-with-dwz and
cc-with-dwz-m.

In addition, tested test-case with target boards cc-with-gdb-index.exp,
cc-with-debug-names.exp and readnow.exp.

gdb/ChangeLog:

2020-04-14  Simon Marchi  <simon.marchi@polymtl.ca>
	    Tom de Vries  <tdevries@suse.de>

	PR symtab/25718
	* psympriv.h (struct partial_symtab::read_symtab)
	(struct partial_symtab::expand_psymtab)
	(struct partial_symtab::read_dependencies): Update comments.
	* dwarf2/read.c (struct dwarf2_include_psymtab::read_symtab): Call
	read_symtab for includer.
	(struct dwarf2_include_psymtab::expand_psymtab): Assert false.
	(struct dwarf2_include_psymtab::readin_p): Call readin_p () for includer.
	(struct dwarf2_include_psymtab::m_readin): Remove.
	(struct dwarf2_include_psymtab::includer): New member function.
	(dwarf2_psymtab::expand_psymtab): Assert !readin.

gdb/testsuite/ChangeLog:

2020-04-14  Tom de Vries  <tdevries@suse.de>

	PR symtab/25718
	* gdb.dwarf2/dw2-symtab-includes.exp: New file.