[FYI] Fix gdb.rust/simple.rs for more recent compilers

[Tom Tromey]

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 <***@tromey.com>

* gdb.rust/simple.rs: Don't initialize empty_enum_value.
---
gdb/testsuite/ChangeLog | 4 ++++
gdb/testsuite/gdb.rust/simple.rs | 2 +-
2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 9acaa79e2b2..e7693a07f54 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,7 @@
[phone] Tom Tromey <***@tromey.com>
+
+ * gdb.rust/simple.rs: Don't initialize empty_enum_value.
+
2018-11-19 Pedro Alves <***@redhat.com>

* gdb.base/warning.exp: Don't override INTERNAL_FLAGS. Use
diff --git a/gdb/testsuite/gdb.rust/simple.rs b/gdb/testsuite/gdb.rust/simple.rs
index 00a25e0828d..79d463c1989 100644
--- a/gdb/testsuite/gdb.rust/simple.rs
+++ b/gdb/testsuite/gdb.rust/simple.rs
@@ -170,7 +170,7 @@ fn main () {
let u = Union { f2: 255 };
let simplelayout = SimpleLayout { f1: 8, f2: 9 };

- let empty_enum_value: EmptyEnum = unsafe { ::std::mem::zeroed() };
+ let empty_enum_value: EmptyEnum;

println!("{}, {}", x.0, x.1); // set breakpoint here
println!("{}", diff2(92, 45));