[IRGen] Update coverage mapping for version 6
The coverage format version we use is automatically
set to whatever the latest version LLVM has. This
resulting in us outputting version 6 as our
format without having made the changes necessary
to reflect the new format. Update the emission
logic to take account for [the new changes in
version 6][1] (we need to include the current
working directory as the first element of the list
of files we output). Additionally, add a
`static_assert` so we don't get caught out by
version bumps in the future. Note we can't pin our
version, as it must stay in sync with the version
Clang is using.
[1]: https://llvm.org/docs/CoverageMappingFormat.html#llvm-ir-representation
