Fix data race with interface caching

src/interface.h

Index: src/interface.h
diff --git a/src/interface.h b/src/interface.h
index 598d0381427095c4c9f31de6365385757ac328b0..ae549634c55a020fd6c4bd65cabe6bdb9ebe8007 100644
--- a/src/interface.h
+++ b/src/interface.h
@@ -41,15 +41,9 @@ class Interface : public ZoneObject {
     return new(zone) Interface(NONE);
   }
 
-  static Interface* NewValue() {
-    static Interface value_interface(VALUE + FROZEN);  // Cached.
-    return &value_interface;
-  }
+  static Interface* NewValue();
 
-  static Interface* NewConst() {
-    static Interface value_interface(VALUE + CONST + FROZEN);  // Cached.
-    return &value_interface;
-  }
+  static Interface* NewConst();
 
   static Interface* NewModule(Zone* zone) {
     return new(zone) Interface(MODULE);
@@ -178,6 +172,9 @@ class Interface : public ZoneObject {
   // ---------------------------------------------------------------------------
   // Implementation.
  private:
+  struct ValueCreate;
+  struct ConstCreate;
+
   enum Flags {    // All flags are monotonic
     NONE = 0,
     VALUE = 1,    // This type describes a value