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>

gdbsupport/common-utils.h

@@ -26,7 +26,7 @@
 #include "gdbsupport/gdb_unique_ptr.h"
 #include "gdbsupport/array-view.h"
 #include "poison.h"
-#include "gdb_string_view.h"
+#include <string_view>
 
 #if defined HAVE_LIBXXHASH
 #  include <xxhash.h>
@@ -94,7 +94,7 @@ extern const char *safe_strerror (int);
    true if the start of STRING matches PATTERN, false otherwise.  */
 
 static inline bool
-startswith (gdb::string_view string, gdb::string_view pattern)
+startswith (std::string_view string, std::string_view pattern)
 {
   return (string.length () >= pattern.length ()
 	  && strncmp (string.data (), pattern.data (), pattern.length ()) == 0);
@@ -228,7 +228,7 @@ fast_hash (const void *ptr, size_t len, unsigned int start_value = 0)
 namespace gdb
 {
 
-/* Hash type for gdb::string_view.
+/* Hash type for std::string_view.
 
    Even after we switch to C++17 and dump our string_view implementation, we
    might want to keep this hash implementation if it's faster than std::hash
@@ -236,7 +236,7 @@ namespace gdb
 
 struct string_view_hash
 {
-  std::size_t operator() (gdb::string_view view) const
+  std::size_t operator() (std::string_view view) const
   {  return fast_hash (view.data (), view.length ()); }
 };