Extract method for CxxSourceRuleFactory
Extract method for CxxSourceRuleFactory

Summary: We want to re-use the `CxxSourceRuleFactory` which is used to create Cxx rules for the diagnostics rules as well. Here, we just extract the logic which creates it, so it can be re-used.

Reviewed By: cjhopman

shipit-source-id: 8a74f3791ba636854e7b78a53441042114b7b0d4

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