[alln] Opt in to -Wno-implicit-int-float-conversion

Otherwise when we turn on this warning project-wide we get this sad
failure:

../../third_party/icu/source/i18n/plurrule.cpp:1664:24: error: implicit conversion from 'int64_t' (aka 'long') to 'double' changes value from 9223372036854775807 to 9223372036854775808 [-Werror,-Wimplicit-int-float-conversion]
          if (scaled > U_INT64_MAX) {
                     ~ ^~~~~~~~~~~
../../third_party/icu/source/common/unicode/umachine.h:212:33: note: expanded from macro 'U_INT64_MAX'
                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Bug: 3203
Change-Id: I8e6984169120547460e3e35a566272e13ce4506c