Remove current_cc_toolchain and all uses

The special cc toolchain alias `@bazel_tools//tools/cpp:current_cc_toolchain` is incorrect and unsafe, specifically in an environment with multiple execution platforms.

Please read [this document](https://docs.google.com/document/d/14vxMd3rTpzAwUI9ng1km1mp-7MrVeyGFnNbXKF_XhAM/edit?usp=sharing) for details.

Much of this work has already been completed inside google's monorepo, this is a tracking issue for public changes needed.