This reverts part of #4038 which made the compiler consider it to be an `Explicit` conformance, breaking source code that was accepted in Swift 3.0 which declared a raw type as well as explicit conformance to `RawRepresentable` (reported as rdar://problem/30386658). While I'm here, a couple of spot fixes:
- Ensure an enum's raw value exprs are type-checked before checking conformances of any of its extensions, since the RawRepresentable conformance derivation will blow up if the raw value exprs haven't been checked. Fixes an order dependency issue if `extension Foo: RawRepresentable {}` gets checked before `enum Foo: Int { ... }`.
- Don't display the custom `enum_declares_rawrep_with_raw_type` diagnostic if the source location for the enum's inheritance clause is invalid, so that we don't emit a dislocated diagnostic.
