DO NOT MERGE Cherry-pick: ICU-20536 Japanese era Reiwa support in ICU4C 58

Cherry-pick https://github.com/unicode-org/icu/commit/76d09144a605d66adfdfd5519376598e4a8840fd
Partial cherry-pick https://github.com/unicode-org/icu/commit/14a8ceb46a5773645ac140080b0b0c02106e98b5
- Exclude non-test files

Bug: 138560701
Test: CtsIcuTestCases
Test: CtsLibcoreTestCases
Test: CtsLibcoreOjTestCases
Change-Id: I9fb956a4f82f7ea126c304b2b09343dd78c76553
4 files changed