llvm-cov: Implement the preserve-paths flag

Until now, when a path in a gcno file included a directory, we would
emit our .gcov file in that directory, whereas gcov always emits the
file in the current directory. In doing so, this implements gcov's
strange name-mangling -p flag, which is needed to avoid clobbering
files when two with the same name exist in different directories.

The path mangling is a bit ugly and only handles unix-like paths, but
it's simple, and it doesn't make any guesses as to how it should
behave outside of what gcov documents. If we decide this should be
cross platform later, we can consider the compatibility implications
then.