gdb: have gdb_disassemble_info carry 'this' in its stream pointer

The gdb_disassemble_info class is a wrapper around the libopcodes
disassemble_info struct.  The 'stream' field of disassemble_info is
passed as an argument to the fprintf_func and fprintf_styled_func
callbacks when the disassembler wants to print anything.

Previously, GDB would store a pointer to a ui_file object in the
'stream' field, then, when the disassembler wanted to print anything,
the content would be written to the ui_file object.  An example of an
fprintf_func callback, from gdb/disasm.c is:

  int
  gdb_disassembler::dis_asm_fprintf (void *stream, const char *format, ...)
  {
    /* Write output to STREAM here.  */
  }

This is fine, but has one limitation, within the print callbacks we
only have access to STREAM, we can't access any additional state
stored within the gdb_disassemble_info object.

Right now this isn't a problem, but in a future commit this will
become an issue, how we style the output being written to STREAM will
depend on the state of the gdb_disassemble_info object, and this state
might need to be updated, depending on what is being printed.

In this commit I propose changing the 'stream' field of the
disassemble_info to carry a pointer to the gdb_disassemble_info
sub-class, rather than the stream itself.

We then have the two sub-classes of gdb_disassemble_info to consider,
the gdb_non_printing_disassembler class never cared about the stream,
previously, for this class, the stream was nullptr.  With the change
to make stream be a gdb_disassemble_info pointer, no further updates
are needed for gdb_non_printing_disassembler.

The other sub-class is gdb_printing_disassembler.  In this case the
sub-class now carries around a pointer to the stream object.  The
print callbacks are updated to cast the incoming stream object back to
a gdb_printing_disassembler, and then extract the stream.

This is purely a refactoring commit.  A later commit will add
additional state to the gdb_printing_disassembler, and update the
print callbacks to access this state.

There should be no user visible changes after this commit.

gdb/disasm.c

@@ -163,17 +163,33 @@ gdb_disassembler::dis_asm_print_address (bfd_vma addr,
   print_address (self->arch (), addr, self->stream ());
 }
 
+/* See disasm.h.  */
+
+ui_file *
+gdb_printing_disassembler::stream_from_gdb_disassemble_info (void *dis_info)
+{
+  gdb_disassemble_info *di = (gdb_disassemble_info *) dis_info;
+  gdb_printing_disassembler *dis
+    = dynamic_cast<gdb_printing_disassembler *> (di);
+  gdb_assert (dis != nullptr);
+  ui_file *stream = dis->stream ();
+  gdb_assert (stream != nullptr);
+  return stream;
+}
+
 /* Format disassembler output to STREAM.  */
 
 int
-gdb_printing_disassembler::fprintf_func (void *stream,
+gdb_printing_disassembler::fprintf_func (void *dis_info,
 					 const char *format, ...)
 {
-  va_list args;
+  ui_file *stream = stream_from_gdb_disassemble_info (dis_info);
 
+  va_list args;
   va_start (args, format);
-  gdb_vprintf ((struct ui_file *) stream, format, args);
+  gdb_vprintf (stream, format, args);
   va_end (args);
+
   /* Something non -ve.  */
   return 0;
 }
@@ -181,14 +197,15 @@ gdb_printing_disassembler::fprintf_func (void *stream,
 /* See disasm.h.  */
 
 int
-gdb_printing_disassembler::fprintf_styled_func (void *stream,
+gdb_printing_disassembler::fprintf_styled_func (void *dis_info,
 						enum disassembler_style style,
 						const char *format, ...)
 {
-  va_list args;
+  ui_file *stream = stream_from_gdb_disassemble_info (dis_info);
 
+  va_list args;
   va_start (args, format);
-  gdb_vprintf ((struct ui_file *) stream, format, args);
+  gdb_vprintf (stream, format, args);
   va_end (args);
   /* Something non -ve.  */
   return 0;
@@ -809,7 +826,7 @@ gdb_disassembler::gdb_disassembler (struct gdbarch *gdbarch,
 /* See disasm.h.  */
 
 gdb_disassemble_info::gdb_disassemble_info
-  (struct gdbarch *gdbarch, struct ui_file *stream,
+  (struct gdbarch *gdbarch,
    read_memory_ftype read_memory_func, memory_error_ftype memory_error_func,
    print_address_ftype print_address_func, fprintf_ftype fprintf_func,
    fprintf_styled_ftype fprintf_styled_func)
@@ -817,7 +834,7 @@ gdb_disassemble_info::gdb_disassemble_info
 {
   gdb_assert (fprintf_func != nullptr);
   gdb_assert (fprintf_styled_func != nullptr);
-  init_disassemble_info (&m_di, stream, fprintf_func,
+  init_disassemble_info (&m_di, (void *) this, fprintf_func,
 			 fprintf_styled_func);
   m_di.flavour = bfd_target_unknown_flavour;
 
@@ -930,7 +947,7 @@ gdb_disassembler::print_insn (CORE_ADDR memaddr,
   /* Push any disassemble output to the real destination stream.  We do
      this even if the disassembler reported failure (-1) as the
      disassembler may have printed something to its output stream.  */
-  m_di.fprintf_func (m_dest, "%s", m_buffer.c_str ());
+  gdb_printf (m_dest, "%s", m_buffer.c_str ());
 
   /* If the disassembler failed then report an appropriate error.  */
   if (length < 0)