gdb: add overloads of gdb_tilde_expand

Like the previous commit, add two overloads of gdb_tilde_expand, one
takes std::string and other takes gdb::unique_xmalloc_ptr<char>.  Make
use of these overloads throughout GDB and gdbserver.

There should be no user visible changes after this commit.

Approved-By: Tom Tromey <tom@tromey.com>