[automerger skipped] DO NOT MERGE Cherry-pick: ICU-20536 Japanese era Reiwa support in ICU4C 56 am: 1aa02d75f9 -s ours am: ceb8b7da47 am: 479c44f420
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Change-Id: Iade23aac1d2217d0bfb8cebac896c9b113a4fb74