Ignore pass-failed warnings on coverage builds.

Clang coverage can disable certain passes, which throws a warning. Some
modules treat all warnings as errors, causing them to fail to build with
Clang coverage. Disable this warning only on coverage builds, as it is
still a useful warning for non-coverage builds.

Bug: 156609447
Test: m CLANG_COVERAGE=true COVERAGE_PATHS='*' libyuv
Change-Id: I1146befc382931f7eb0f1145ec63b90757600b61