Do not build with Werror by default (Bazel build)

This seems like an anti-pattern to have -Werror on by default:
it is hostile to user as we can't ensure that all of the supported
platforms are warning-free, and any newer compiler could break the build
for a user who does not have a clear actionable way around it.

Reviewed By: GMNGeoffrey, kuhar

Differential Revision: https://reviews.llvm.org/D123481