ICU-21125 Improvements to resource fallback:

ICU-21125 Improvements to resource fallback:

- Added code to use the parentLocales data in supplementalData.xml to determine the "parent locale ID" to use when
  the requested resource bundle is not present (ICU-21126).
- Added code to change the parent-chain search path to handle the script better (ICU-21125; algorithm was described
  in CLDR-15265):
  - The base search patch is now ll_Ssss_RR -> ll_RR -> ll_Ssss -> ll -> root
  - If the requested script is not the default script for the requested language and region, we automatically
    avoid fallbacks that will implicitly change the script.
- Added new code to the CLDR-to-ICU data generation tool to generate source code, and used it to generate the lookup
  tables for the new resource-fallback logic (we can't use the existing resource files for this, since that would
  involve opening a resource bundle while trying to open another resource bundle).  The data-generation stuff is
  intended to be generic enough to allow for us to generate more static data tables in the future.
- Commented out a few collator tests, and changed one resource bundle test, because they're incompatible with the
  new fallback logic (specifically, the default-script logic).