[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: 51b3cfa897 -s ours am: 1d131eda18 -s ours am: 71fba88687
am: 39b1d9a6a1

Change-Id: I8461cd5bc9c3bcb6cdafa661fcb1fa946666f668