Use rust_demangle to fix a crash

Use rust_demangle to fix a crash

PR rust/30211 points out a crash caused by a particular completion.
This turns out to happen because a Rust minsym winds up in a
C++-specific path in strncmp_iw_with_mode, which ultimately causes the
completer to pass invalid arguments to string::append.

This patch fixes the bug by reordering the language constants so that
Rust comes before C++, and then using rust_demangle.  This ensures
that minsyms are correctly marked as "Rust", avoiding this code and
thus the crash.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=20367
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30211
Reviewed-By: Andrew Burgess <aburgess@redhat.com>