Create a directory for --out-dir if it does not already exist

Create a directory for --out-dir if it does not already exist

Currently if `--out-dir` is set to a non-existent directory, the
compiler will throw unfriendly messages like `error: could not write
output to subdir/example.crate.allocator.rcgu.o: No such file or
directory`, which, while not completely unreadable, isn’t very
user-friendly either. This change creates the directory automatically
if it does not yet exist.