Fix gdb.rust/simple.rs for more recent compilers

Fix gdb.rust/simple.rs for more recent compilers

gdb.rust/simple.exp will fail when run with a recent version of rustc.
This patch fixes the test case so that it will continue to run.

Tested on x86-64 Fedora 28.

I also temporarily backed out the rust-lang.c from
commit 098b2108a2b61531c0bc8ea16854f773083a95d7, and verified that
this updated test still would have provoked the original bug.

gdb/testsuite/ChangeLog
2018-11-19  Tom Tromey  <tom@tromey.com>

	* gdb.rust/simple.rs: Don't initialize empty_enum_value.