[AST] NFC: Avoid fall through warnings in non-assert builds

Author: davezarzycki

include/swift/AST/Ownership.h

@@ -45,22 +45,24 @@ enum : unsigned { NumReferenceOwnershipBits =
 static inline llvm::StringRef keywordOf(ReferenceOwnership ownership) {
   switch (ownership) {
   case ReferenceOwnership::Strong:
-    assert(false && "not applicable");
+    break;
   case ReferenceOwnership::Weak: return "weak";
   case ReferenceOwnership::Unowned: return "unowned";
   case ReferenceOwnership::Unmanaged: return "unowned(unsafe)";
   }
+  // We cannot use llvm_unreachable() because this is used by the stdlib.
   assert(false && "impossible");
 }
 
 static inline llvm::StringRef manglingOf(ReferenceOwnership ownership) {
   switch (ownership) {
   case ReferenceOwnership::Strong:
-    assert(false && "not applicable");
+    break;
   case ReferenceOwnership::Weak: return "Xw";
   case ReferenceOwnership::Unowned: return "Xo";
   case ReferenceOwnership::Unmanaged: return "Xu";
   }
+  // We cannot use llvm_unreachable() because this is used by the stdlib.
   assert(false && "impossible");
 }