gdb: Remove uses of gdb::to_string (const std::string_view &)

gdb: Remove uses of gdb::to_string (const std::string_view &)

This patch removes all uses of to_string(const std::string_view&) and
use the std::string ctor or implicit conversion from std::string_view to
std::string instead.

A later patch will remove this gdb::to_string while removing
gdbsupport/gdb_string_view.h.

Change-Id: I877cde557a0727be7b0435107e3c7a2aac165895
Approved-By: Tom Tromey <tom@tromey.com>
Approved-By: Pedro Alves <pedro@palves.net>