Modules: Document that ReadASTCore exits its final loop via `return`, NFC

The final loop never breaks.  Document that by following it with
llvm_unreachable.

llvm-svn: 355294

clang/lib/Serialization/ASTReader.cpp

@@ -4257,7 +4257,7 @@ ASTReader::ReadASTCore(StringRef FileName,
     }
   }
 
-  return Success;
+  llvm_unreachable("unexpected break; expected return");
 }
 
 ASTReader::ASTReadResult