Cleanup -Wunused-but-set-variable in warnings

Clang extended Wunused-but-set-variable warning for pre/post unary
operators since https://github.com/llvm/llvm-project/commit/460fc440ad8d.
This patch cleans up chromium's copy until the issue is resolved
in the upstream:
https://unicode-org.atlassian.net/browse/ICU-21966
https://github.com/unicode-org/icu/pull/2055

Change-Id: If572f017403663071a19bc1dca45e7befba7b7e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/deps/icu/+/3553653
Reviewed-by: Frank Tang <ftang@chromium.org>
5 files changed