Migrate users of C++ toolchain API for --incompatible_disable_depset_in_cc_user_flags
Migrate users of C++ toolchain API for --incompatible_disable_depset_in_cc_user_flags

This is a migration needed do move g3 forward so unknown commit can be submitted,
and in case of Bazel users, to make them forward compatible with Bazel 0.20.
Migration details and discussion are over at github:

https://github.com/bazelbuild/bazel/issues/6383

PiperOrigin-RevId: 217837227

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