RelatedMethodFinder.GetBaseMethod should use internal reflection mechanism for runtime methods (like InheritanceAwareCustomAttributeRetriever)

In addition, for mutable methods, it can use MutableMethodInfo.BaseMethod. (In fact, since we don't expect other kinds of methods anyway, the method finder could simply throw for other MethodInfos and the complex implementation using GetMostDerivedOverride can be removed.)

Impact in Debug-Builds: ~3%

NOTE: Needs case for CustomMethodInfo, possibly fallback to original implementation?