[build] Suppress -Wenum-constexpr-conversion

Clang recently added a warning for values outside enum range in
https://reviews.llvm.org/D131307 and https://reviews.llvm.org/D130058.
This triggered a warning in icu, which is reported in
https://unicode-org.atlassian.net/browse/ICU-22113

This patch temporarily suppresses -Wunknown-warning-option and
-Wenum-constexpr-conversion in Fucshia to fix the builders.

Change-Id: I5c89e3a22cf52145232bfcd001ac3495daafa287
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/deps/icu/+/3821449
Reviewed-by: Jungshik Shin <jshin@chromium.org>
1 file changed