When we try to extract coverage-relevant spans from MIR, sometimes we see MIR
statements/terminators whose spans cover the entire function body. Those spans
tend to be unhelpful for coverage purposes, because they often represent
compiler-inserted code, e.g. the implicit return value of `()`.
Without the workaround applied, this test will produce malformed mappings that
cause `llvm-cov` to fail.
(And if it does emit well-formed mappings, they should be obviously incorrect.)