gdb: add gdb_rl_tilde_expand util

gdb: add gdb_rl_tilde_expand util

Add gdb_rl_tilde_expand, a wrapper around readline's tilde_expand that
returns a gdb::unique_xmalloc_ptr<char>.  Change all callers of
tilde_expand to use gdb_rl_tilde_expand (even the couple of spots that
release it immediatly, for consistency).  This simplifies a few callers.

The name gdb_tilde_expand is already taken by a home-made implementation
in gdbsupport/gdb_tilde_expand.{h.cc}.  I wonder if we could just use
that one instead of readline's tilde_expand, but that's an orthogonal
question.  I don't know how they differ, and I don't want to introduce
behavior changes in this patch.

Change-Id: I6d34eef19f86473226df4ae56d07dc01912e3131
Approved-By: Tom Tromey <tom@tromey.com>