Verifier: accept enums as scopes

Rust allows enums to be scopes, as shown by the previous change. Sadly,
D111770 disallowed enums-as-scopes in the LLVM Verifier, which means
that LLVM HEAD stopped working for Rust compiles.  As a result, we back
out the verifier part of D111770 with a modification to the testcase so
we don't break this in the future.

The testcase is now actual IR from rustc at commit 8f8092cc3, which is
the nightly as of 2021-09-28. I would expect rustc 1.57 to produce
similar or identical IR if someone wants to reproduce this IR in the
future with minimal changes. A recipe for reproducing the IR using rustc
is included in the test file.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D115353