Fix warnings raised by clang 11 (#8664)
Fix warnings raised by clang 11 (#8664)

This fixes a few kinds of warnings:
 - inconsistent-missing-override when the overriding site is missing the
   override keyword
 - unused-function when a function is neither visible nor used
 - unused-private-field when a private field exists but is never used
 - sign-compare when unsigned ints are compared to signed ints. Not all
   of those have been addressed, but this warning isn't enabled by
   default.

This fixes #8612.
protocolbuffers/protobuf-javascript,protocolbuffers/protobuf-javascript,protocolbuffers/protobuf-javascript,protocolbuffers/protobuf-javascript