fixup mtype.h for dlang#20946 (dlang#20971)

Author: thewilsonator

compiler/src/dmd/mtype.h

@@ -554,6 +554,8 @@ class TypeFunction final : public TypeNext
     void isReturn(bool v);
     bool isReturnScope() const;
     void isReturnScope(bool v);
+    bool isRvalue() const;
+    void isRvalue(bool v);
     bool isScopeQual() const;
     void isScopeQual(bool v);
     bool isReturnInferred() const;