Implement icu4c LocaleBuilder
Description
This is the tracking bug for the implementation of icu4c LocaleBuilder
a. the API was reviewed & approved by ICU-TC on 2018-10-31, target for r64
see https://goo.gl/SeJNpU
b. Design doc at https://goo.gl/Qf12p3
c. Implementation at https://github.com/FrankYFTang/icu/tree/localeBuilder
