When reading DBGSourcePathRemapping from a dSYM, we remove the last two
path components to make the source lookup more general. However, when
dealing with a relative path that has less than 2 components, we ended
up with an invalid (empty) FileSpec.
This patch changes the behavior of removeLastPathComponent to remove the
last path component, if possible. It does this by checking whether a
parent path exists, and if so using that as the new path. We rely
entirely on LLVM's path implementation to do the heavy lifting.
We now also return a boolean which indicates whether the operator was
successful or not.
Why add this? If the path is just "." to begin with, there is nothing to do. I would vote to not add this argument since we don't need it. Add it back if we ever do.