Make it possible to use llvm-cov with `bazel coverage`

[mkosiba]

The fact that llvm-cov and llvm-profdata are not set seems to trip up the external/bazel_tools/tools/test/collect_cc_coverage.sh script resulting in empty .dat files.

To solve the issue we implement something similar to what's done in rules_cc.

After these changes bazel coverage <test target> no longer produces empty .dat files.

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