CP PR2090 to remove ATOMIC_VAR_INIT

Remove macros  ATOMIC_INT32_T_INITIALIZER and U_INITONCE_INITIALIZER
for C++20 because ATOMIC_VAR_INIT is removed from C++20.

Bug: chromium:1284275
Change-Id: Iedf777df164d60ffb801ea20440b2583b44bad0d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/deps/icu/+/3652613
Reviewed-by: Jungshik Shin <jshin@chromium.org>
67 files changed