Avoid running one Rust test against older LLVM

LLVM 8.0 introduced some changes to let the Rust compiler emit DWARF
variant parts.  Before this change, the compiler would emit two types
with the same name, and unfortunately gdb happens to pick the wrong
one.  So, this patch disables the test when using an older version of
LLVM.

gdb/testsuite/ChangeLog
2020-09-15  Tom Tromey  <tromey@adacore.com>

	PR rust/26197:
	* lib/rust-support.exp (rust_llvm_version): New proc.
	* gdb.rust/simple.exp: Check rust_llvm_version.