In observing spurious cache hits for -fplugin=foo.so when foo.so changed we realized foo.so was not in the extra hash files.

The root cause here is that cmp does not account for arguments declared CanBeConcatenated like fplugin.

This can be observed in this failing test case: https://github.com/2over12/sccache/blob/6e423c58d8806bafb4dcb42137f330e61b991450/src/compiler/clang.rs#L938

I also dont think -fplugin x is valid for clang so this could potentially be Concatenated