Previously the path to covered files in the __LLVM_COV / __llvm_covmap
section were absolute. This made remote builds with coverage information
difficult because all machines would have to have the same build root.
This change uses the values for `-coverage-prefix-map` to remap files in
the coverage info to relative paths. These paths work correctly with
llvm-cov when it is run from the same source directory as the
compilation, or from a different directory using the `-path-equivalence`
argument.
This is analogous to this change in clang https://reviews.llvm.org/D81122
