Issue #2142 - Implement syntax for public/private fields and computed field names

This state still has the initializers scoped on .initializers local variable, which will be changed later.

Based-on: m-c 1499448, 1530084, 1530832, 1529448 (partial), 1532921, 1528039, 1528038, 1535166, 1550628,
              1535166, 1550628, 1541641, 1547133, 1540787, 1535804/9

js/src/jsscript.h

@@ -1999,11 +1999,39 @@ static_assert(sizeof(JSScript) % js::gc::CellSize == 0,
 
 namespace js {
 
+struct FieldInitializers
+{
+#ifdef DEBUG
+    bool valid;
+#endif
+    // This struct will eventually have a vector of constant values for optimizing
+    // field initializers.
+    size_t numFieldInitializers;
+
+    explicit FieldInitializers(size_t numFieldInitializers)
+      :
+#ifdef DEBUG
+        valid(true),
+#endif
+        numFieldInitializers(numFieldInitializers) {
+    }
+
+    static FieldInitializers Invalid() { return FieldInitializers(); }
+
+  private:
+    FieldInitializers()
+      :
+#ifdef DEBUG
+        valid(false),
+#endif
+        numFieldInitializers(0) {
+    }
+};
+
 // Information about a script which may be (or has been) lazily compiled to
 // bytecode from its source.
 class LazyScript : public gc::TenuredCell
 {
-  private:
     // If non-nullptr, the script has been compiled and this is a forwarding
     // pointer to the result. This is a weak pointer: after relazification, we
     // can collect the script if there are no other pointers to it.
@@ -2030,7 +2058,6 @@ class LazyScript : public gc::TenuredCell
     uint32_t padding;
 #endif
 
-  private:
     static const uint32_t NumClosedOverBindingsBits = 20;
     static const uint32_t NumInnerFunctionsBits = 20;
 
@@ -2072,6 +2099,8 @@ class LazyScript : public gc::TenuredCell
         uint64_t packedFields_;
     };
 
+    FieldInitializers fieldInitializers_;
+
     // Source location for the script.
     // See the comment in JSScript for the details.
     uint32_t begin_;
@@ -2297,6 +2326,12 @@ class LazyScript : public gc::TenuredCell
         p_.hasThisBinding = true;
     }
 
+    void setFieldInitializers(FieldInitializers fieldInitializers) {
+        fieldInitializers_ = fieldInitializers;
+    }
+
+    FieldInitializers getFieldInitializers() const { return fieldInitializers_; }
+
     const char* filename() const {
         return scriptSource()->filename();
     }