Merge pull request #346 from dwrensha/asan

prevent use-after-free in internal compiler API

c++/src/capnp/compiler/compiler.c++

@@ -320,12 +320,13 @@ private:
   kj::Arena nodeArena;
   // Arena used to allocate nodes and other permanent objects.
 
-  Workspace workspace;
-  // The temporary workspace.
-
   std::unordered_map<Module*, kj::Own<CompiledModule>> modules;
   // Map of parser modules to compiler modules.
 
+  Workspace workspace;
+  // The temporary workspace. This field must be declared after `modules` because objects
+  // allocated in the workspace may hold references to the compiled modules in `modules`.
+
   std::unordered_map<uint64_t, Node*> nodesById;
   // Map of nodes by ID.