[Coverage] Support overriding compilation directory

When making compilation relocatable, for example in distributed
compilation scenarios, we want to set compilation dir to a relative
value like `.` but this presents a problem when generating reports
because if the file path is relative as well, for example `..`, you
may end up writing files outside of the output directory.

This change introduces a flag that allows overriding the compilation
directory that's stored inside the profile with a different value that
is absolute.

Differential Revision: https://reviews.llvm.org/D100232