NCGenerics: remove unreachable and explain

Author: kavon

lib/Sema/TypeCheckInvertible.cpp

@@ -468,13 +468,9 @@ ProtocolConformance *deriveConformanceForInvertible(Evaluator &evaluator,
     // All types already start with conformances to the invertible protocols in
     // this case, within `NominalTypeDecl::prepareConformanceTable`.
     //
-    // I'm currently unsure what happens when rebuilding a module from its
-    // interface, so this might not be unreachable code just yet.
-    if (SWIFT_ENABLE_EXPERIMENTAL_NONCOPYABLE_GENERICS &&
-        file->getKind() != FileUnitKind::Synthesized) {
-      llvm_unreachable("when can this actually happen??");
-    }
-
+    // There are various other kinds of SourceFiles, like SIL, which instead
+    // get their conformances here instead.
+    //
     // If there's no inverse, we infer a positive IP conformance.
     return generateConformance(nominal);
   }