shims/super/cc: relax restrictions with -Xclang

[Bo98]

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For a lot of cases, -Xpreprocessor and -Xclang do similar things. We already have relaxed restrictions for -Xpreprocessor so we should give equal treatment to -Xclang.

Notably, the comment explicitly mentions -Xpreprocessor -fopenmp but -Xclang -fopenmp is equally valid and is used within CMake.

Fixes Homebrew/homebrew-core#53598 (comment). With this change, all the added args in that pull request can be removed again.