ICU-13778 DataTimePatternGenerator code refactor. Handle Out-of-Memor…

…y (OOM) errors, use LocalPointer to prevent memory leaks when OOM occurs, use an internal error code to better report errors during clone and copy construction, mark helper methods and parameters as const, use nullptr instead of NULL, minor spelling and formatting changes. (Note: All tests pass on Windows and Linux).

X-SVN-Rev: 41552