Reverse accidental src/llvm reversion in 876483dcf, and add test.

The test is reduced from a doc test, but making it separate ensures that
(1) unrelated changes to the docs won't leave this case uncovered, and
(2) the nature of any future failures will be more obvious to whoever
sees the tree on fire as a result.
Revert "doc: XFAIL mysterious failure on bots"

This reverts commit 2c6dae081b72d33a2c51b5f46436e6350bb3682f.
auto merge of #6136 : jld/rust/simplifycfg-unrevert, r=thestinger

Also add the clearly marked test case that should have been there all along, and un-xfail the not-so-obvious doc test that was affected.