gdb: Use std::string_view instead of gdb::string_view

Given that GDB now requires a C++17, replace all uses of
gdb::string_view with std::string_view.

This change has mostly been done automatically:
- gdb::string_view -> std::string_view
- #include "gdbsupport/gdb_string_view.h" -> #include <string_view>

One things which got brought up during review is that gdb::stging_view
does support being built from "nullptr" while std::sting_view does not.
Two places are manually adjusted to account for this difference:
gdb/tui/tui-io.c:tui_getc_1 and
gdbsupport/format.h:format_piece::format_piece.

The above automatic change transformed
"gdb::to_string (const gdb::string_view &)" into
"gdb::to_string (const std::string_view &)".  The various direct users
of this function are now explicitly including
"gdbsupport/gdb_string_view.h".  A later patch will remove the users of
gdb::to_string.

The implementation and tests of gdb::string_view are unchanged, they will
be removed in a following patch.

Change-Id: Ibb806a7e9c79eb16a55c87c6e41ad396fecf0207
Approved-By: Tom Tromey <tom@tromey.com>
Approved-By: Pedro Alves <pedro@palves.net>

2 files changed, 7 insertions, 4 deletions

diff --git a/gdb/tui/tui-io.c b/gdb/tui/tui-io.c
index b8954af..8472464 100644
--- a/gdb/tui/tui-io.c
+++ b/gdb/tui/tui-io.c
@@ -1197,7 +1197,10 @@ tui_getc_1 (FILE *fp)
 	 Compare keyname instead.  */
       if (ch >= KEY_MAX)
 	{
-	  auto name = gdb::string_view (keyname (ch));
+	  std::string_view name;
+	  const char *name_str = keyname (ch);
+	  if (name_str != nullptr)
+	    name = std::string_view (name_str);
 
 	  /* The following sequences are hardcoded in readline as
 	     well.  */
diff --git a/gdb/tui/tui-win.c b/gdb/tui/tui-win.c
index a0a1233..9219c1f 100644
--- a/gdb/tui/tui-win.c
+++ b/gdb/tui/tui-win.c
@@ -54,7 +54,7 @@
 #include "gdb_curses.h"
 #include <ctype.h>
 #include "readline/readline.h"
-#include "gdbsupport/gdb_string_view.h"
+#include <string_view>
 
 #include <signal.h>
 
@@ -663,7 +663,7 @@ tui_scroll_right_command (const char *arg, int from_tty)
 
 /* Answer the window represented by name.  */
 static struct tui_win_info *
-tui_partial_win_by_name (gdb::string_view name)
+tui_partial_win_by_name (std::string_view name)
 {
   struct tui_win_info *best = nullptr;
 
@@ -935,7 +935,7 @@ tui_set_win_size (const char *arg, bool set_width_p)
   buf_ptr = skip_to_space (buf_ptr);
 
   /* Validate the window name.  */
-  gdb::string_view wname (buf, buf_ptr - buf);
+  std::string_view wname (buf, buf_ptr - buf);
   win_info = tui_partial_win_by_name (wname);
 
   if (win_info == NULL)