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

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>