Provide an easy way to enable all of the default warning severity checks

[davido]

There is no easy way to enable all of the default warning severity checks. One it's there it could be used for different tool chain integrations, like Bazel's javacopts.

[davido]

@greggdonovan's team added a workaround for this issue for Bazel toolchain, to enable the individual checks in Bazel: [1]. The interesting question is how can we ensure that this warning list is complete? What if next EP release would add yet another 42 warnings, what would be the appropriate trigger to update this list?

@eaftan @cushon Any thoughts?

I'm asking because I've implemented a different approach for all warnings activation in gerrit: [2], and have now concerns that this list is already outdated. BTW, my implementation was inspired by @cushon's comment: [3].

• [1] https://github.com/bazelbuild/BUILD_file_generator/blob/master/tools/bazel_defs/java.bzl
• [2] https://gerrit-review.googlesource.com/c/gerrit/+/194045
• [3] Feature request: Setting default javacopts for all java_library rules in workspace bazelbuild/bazel#3427 (comment)