[automerger skipped] DO NOT MERGE Cherry-pick: ICU-20536 Japanese era Reiwa support in ICU4C 58 am: 07cd4ca6 -s ours am: 33dc1eb9 am: 057adef1 am: c1c5c82f -s ours am: 71b3f220 -s ours am: 11820b97 am: a8375ab9 Change-Id: I5bd720809b2b9557ce11e3af0844db78c16e7fa9