Prototype flag normalisation for GHC 8.0-8.4

[merijn]

Uses the just merged infrastructure to normalise commandline arguments to filter out any GHC flags that don't affect the build products. Currently only filters commandline arguments for GHC >=8.0 && <8.5 as those are the only versions I checked the flags for.

I went through the GHC DynFlags source for all those version to figure out which flags can be filtered and to follow the right format. Anything I wasn't sure about is left alone, some flags I'm still unsure about are documented in #4247.

Also filters out all haddock arguments, as those should never affect GHC build products.

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• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.
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[merijn]

Ok, I've been using this code for about a week now with no issues, none of the people in the original issue or that I've asked have objected, so unless someone has a complaint before tomorrow I'm merging this.