Version 5 adds Branch Regions which are a prerequisite for branch coverage.
Version 6 can use the zeroth filename as prefix for other relative files.

This commit augments Swatinem's initial commit in uncommitted PR rust-lang#90047,
which was a great starting point, but did not fully support LLVM
Coverage Mapping Format version 6.

Version 6 requires adding the compilation directory when file paths are
relative, and since Rustc coverage maps use relative paths, we should
add the expected compilation directory entry.

Note, however, that with the compilation directory, coverage reports
from `llvm-cov show` can now report file names (when the report includes
more than one file) with the full absolute path to the file.

This would be a problem for test results, but the workaround (for the
rust coverage tests) is to include an additional `llvm-cov show`
parameter: `--compilation-dir=.`