[automerger skipped] DO NOT MERGE Cherry-pick: ICU-20536 Japanese era Reiwa support in ICU4C 58 am: 07cd4ca63c -s ours am: 9c52a97ef0 -s ours am: f75a37f4a1 -s ours
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Change-Id: I46ae716d97bff546f2e6f94d26700c64409fb3ec