[ Regression due to upgrade from rustc version 1.32.0 to 1.36.0 ]

On my openSUSE Leap 15.1 laptop I build gdb and ran the gdb testsuite, and ran
into a failure in the gdb.rust testsuite
(https://sourceware.org/bugzilla/show_bug.cgi?id=25048).

The situation on the distro is that (summarizing Tom Tromey's comment here)
with a relatively recent version of Rust but a somewhat older version of LLVM,
the Rust compiler will emit a legacy encoding of enums.

The problem is that there's a bug in the resulting debug info that makes gdb
print incorrect values or "<error reading variable>".

We could get this fixed in the rust compiler, either upstream or locally if
upstream's not interested (which is what Tom Tromy expects), or perhaps make
rustc work with a more recent version of LLVM.

On openSUSE 15.1, we have rustc version 1.36.0 using llvm 7.

On openSUSE tumbleweed (installed in a VM), we have rustc version 1.37.0 and
llvm 8, and there there's no failure.

I tried installing the rustc from devel:languages:rust, and got rustc version
1.38.0 and llvm 7, and the failure was still there.