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Implement cc_toolchain_config rule and cc_legacy_file_group rule.
Note that this rule is in the impl/ directory because we require users to use the cc_toolchain_config rule via the cc_toolchain macro that we will define later, to ensure that parameters such as `compile_files` are passed correctly.
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PiperOrigin-RevId: 612998387
Change-Id: I986d11775e368c4386a930ab2ce8663956a57f9d