Complete switch statement

Author: ApolloZhu

lib/Sema/TypeCheckBitwise.cpp

@@ -299,6 +299,7 @@ bool DeriveImplicitBitwiseCopyableConformance::allowedForFile() {
 
     case SourceFileKind::Library:
     case SourceFileKind::MacroExpansion:
+    case SourceFileKind::DefaultArgument:
     case SourceFileKind::Main:
     case SourceFileKind::SIL:
       return true;