Cherry-pick ICU-20972 Fix invalid conversion from const char8_t* to const char* (C++20)

Cherry-pick upstream commit
https://github.com/unicode-org/icu/commit/3b58179396d9ca6e40a6890d1e5725b9f6d87db9

Test: n/a
Change-Id: I0ee5e2e7d5961e9b61c415bcf84aab7aef2f1e50
7 files changed