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>

gdb/language.c

@@ -641,7 +641,7 @@ default_symbol_name_matcher (const char *symbol_search_name,
 			     const lookup_name_info &lookup_name,
 			     completion_match_result *comp_match_res)
 {
-  gdb::string_view name = lookup_name.name ();
+  std::string_view name = lookup_name.name ();
   completion_match_for_lcd *match_for_lcd
     = (comp_match_res != NULL ? &comp_match_res->match_for_lcd : NULL);
   strncmp_iw_mode mode = (lookup_name.completion_mode ()