Make gdb.lookup_typename work for Rust types

Make gdb.lookup_typename work for Rust types

PR rust/21763 points out that gdb.lookup_typename does not work properly
for (some) Rust types.  I tracked this down to a missing case in
symbol_matches_domain.

Tested by the buildbot.

2017-07-14  Tom Tromey  <tom@tromey.com>

	PR rust/21763:
	* symtab.c (symbol_matches_domain): Add language_rust to special
	case.
	* rust-exp.y (convert_ast_to_expression) <OP_VAR_VALUE>: Don't
	treat LOC_TYPEDEF symbols as variables.

2017-07-14  Tom Tromey  <tom@tromey.com>

	* gdb.rust/simple.exp: Add regression test for PR rust/21763.