[5.1][Runtime] Avoid +class overrides when initializing an ObjC class. #24637
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Cherry-pick #24618 to 5.1.
swift_getInitializedObjCClass called [c class] to trigger class initialization, and returned the value. This wreaked havoc when the class in question overrides +class. Instead, ignore the return value and return c. Switch from +class to +self, which is much less likely to be overridden. Calling an overridden method could have performance downsides or even cause unwanted side effects.
rdar://problem/49853091