Implement fine-grained `-Werror=...` facility

1. ..copied from Design/Warnings:

Introduce variant of -Werror (c.f. GCC's -Werror=*) which allows to specify the individual warnings to be promoted to errors, e.g.

• -Wall -Werror=orphans would only promote -Worphans warnings into errors

• -Wall -Werror -Wno-error=missing-methods would promote all warnings //except// -Wmissing-methods into errors

TODO: specify semantics a bit more exactly (the basic idea is to follow gcc/clang as much as sensible, and only deviate where it's necessary)

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Trac field	Value
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Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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