Fixed crash in DtoCreateNestedContextType().

In some cases, like the following, DtoDeclareFunction() hasn't already been called when DtoCreateNestedContextType() is invoked. This seems to have been anticipated when the function was originally written, but DtoDeclareFunction() was previously called after the ir func was already accessed in fd->ir.irFunc->nestedContextCreated.

---
void main() {
  mixin({
    string foo() {
      return "";
    }

    string bar()() {
      return foo();
    }

    return bar();
  }());
}
---

gen/nested.cpp

@@ -365,12 +365,15 @@ static void DtoCreateNestedContextType(FuncDeclaration* fd) {
     Logger::println("DtoCreateNestedContextType for %s", fd->toChars());
     LOG_SCOPE
 
+#if DMDV2
+    DtoDeclareFunction(fd);
+#endif
+
     if (fd->ir.irFunc->nestedContextCreated)
         return;
     fd->ir.irFunc->nestedContextCreated = true;
 
 #if DMDV2
-    DtoDeclareFunction(fd);
     if (fd->nestedVars.empty()) {
         // fill nestedVars
         size_t nnest = fd->closureVars.dim;
@@ -381,6 +384,7 @@ static void DtoCreateNestedContextType(FuncDeclaration* fd) {
         }
     }
 #endif
+
     if (nestedCtx == NCHybrid) {
         // construct nested variables array
         if (!fd->nestedVars.empty())