gdb: make timestamped_file implement can_emit_style_escape

In our AMDGPU downstream port, we use styling in some logging output.
We noticed it stopped working after the gdb_printf changes.  Making
timestamped_file implement can_emit_style_escape (returning the value of
the stream it wraps) fixes it.  To show that it works, modify some
logging statements in auto-load.c to output style filenames.  You can
see it in action by setting "set debug auto-load 1" and running a
program.  We can incrementally add styling to other debug statements
throughout GDB, as needed.

Change-Id: I78a2fd1e078f80f2263251cf6bc53b3a9de9c17a

gdb/auto-load.c

@@ -733,8 +733,8 @@ auto_load_objfile_script_1 (struct objfile *objfile, const char *realname,
   gdb_file_up input = gdb_fopen_cloexec (filename.c_str (), "r");
   debugfile = filename.c_str ();
 
-  auto_load_debug_printf ("Attempted file \"%s\" %s.",
-			  debugfile,
+  auto_load_debug_printf ("Attempted file \"%ps\" %s.",
+			  styled_string (file_name_style.style (), debugfile),
 			  input != nullptr ? "exists" : "does not exist");
 
   std::string debugfile_holder;
@@ -763,8 +763,9 @@ auto_load_objfile_script_1 (struct objfile *objfile, const char *realname,
 
 	  input = gdb_fopen_cloexec (debugfile, "r");
 
-	  auto_load_debug_printf ("Attempted file \"%s\" %s.",
-				  debugfile,
+	  auto_load_debug_printf ("Attempted file \"%ps\" %s.",
+				  styled_string (file_name_style.style (),
+						 debugfile),
 				  (input != nullptr
 				   ? "exists"
 				   : "does not exist"));

gdb/ui-file.h

@@ -402,6 +402,9 @@ public:
   {
   }
 
+  bool can_emit_style_escape () override
+  { return m_stream->can_emit_style_escape (); }
+
   DISABLE_COPY_AND_ASSIGN (timestamped_file);
 
   void write (const char *buf, long len) override;