[Bazel] Don't fail the build on usage of deprecated APIs

[GMNGeoffrey]

Summary

Build failures are not a particularly helpful way to enforce not using
deprecated APIs and that isn't the point of the Bazel build.

At the same time, this removes -Wno-unused this is a check that we do
enforce in the Google internal build and so are ok maintaining in our
maintenance of the upstream Bazel build (the comment about not wanting
to do so was from a time when this was in a separate repository and I was
the only one maintaining it).

[rupprecht]

SGTM

utils/bazel/.bazelrc

 ###############################################################################
 
 # Set the default compiler to the `clang` binary on the `PATH`.
 build:generic_clang --repo_env=CC=clang
 
 # C++14 standard version is required.
 build:generic_clang --cxxopt=-std=c++14 --host_cxxopt=-std=c++14
 
+# Use `-Wall` and `-Werror` for Clang.
+build:generic_clang --copt=-Wall --copt=-Werror --host_copt=-Wall --host_copt=-Werror
+
 # The Clang available on MacOS has a warning that isn't clean on MLIR code. The
 # warning doesn't show up with more recent Clangs, so just disable for now.
 build:generic_clang --cxxopt=-Wno-range-loop-analysis --host_cxxopt=-Wno-range-loop-analysis
 
-# Use `-Wall` and `-Werror` for Clang.
-build:generic_clang --copt=-Wall --copt=-Werror --host_copt=-Wall --host_copt=-Werror
-# This doesn't appear to be enforced by any upstream bot.
-build:generic_clang --copt=-Wno-unused --host_copt=-Wno-unused
+# Build errors are not a helpful way to enforce deprecation in-repo and it is
+# not the point of the Bazel build to catch usage of deprecated APIs.
+build:generic_clang --copt=-Wno-deprecated --host_copt=-Wno-deprecated
 
 ###############################################################################
 # Options for "generic_gcc" builds: these options should generally apply to
 # builds using a GCC-based compiler, and default to the `gcc` executable on
 # the `PATH`. While these are provided for convenience and may serve as a
 # reference, it would be preferable for users to configure an explicit C++
 # toolchain instead of relying on `.bazelrc` files.
 ###############################################################################
 
 # Set the default compiler to the `gcc` binary on the `PATH`.
 build:generic_gcc --repo_env=CC=gcc
 
 # C++14 standard version is required.
 build:generic_gcc --cxxopt=-std=c++14 --host_cxxopt=-std=c++14
 
+# Build errors are not a helpful way to enforce deprecation in-repo and it is
+# not the point of the Bazel build to catch usage of deprecated APIs.
+build:generic_gcc --copt=-Wno-deprecated --host_copt=-Wno-deprecated
+
 # Disable GCC warnings that are noisy and/or false positives on LLVM code.
 # These need to be global as some code triggering these is in header files.
 build:generic_gcc --copt=-Wno-unused-parameter --host_copt=-Wno-unused-parameter
 build:generic_gcc --copt=-Wno-comment --host_copt=-Wno-comment
 build:generic_gcc --cxxopt=-Wno-class-memaccess --host_cxxopt=-Wno-class-memaccess
 build:generic_gcc --copt=-Wno-maybe-uninitialized --host_copt=-Wno-maybe-uninitialized
 build:generic_gcc --copt=-Wno-misleading-indentation --host_copt=-Wno-misleading-indentation