Simplify remove, create_directory and create_directories.

Before this patch they would take an boolean argument to say if the path
already existed. This was redundant with the returned error_code which is able
to represent that. This allowed for callers to incorrectly check only the
existed flag instead of first checking the error code.

Instead, pass in a boolean flag to say if the previous (non-)existence should be
an error or not.

Callers of the of the old simple versions are not affected. They still ignore
the previous (non-)existence as they did before.

llvm-svn: 201979