Avoid string.clear and string.push_back (#13319)

Compiler "gcc version 2.9-aix51-020209" does not offer them.

Tests/CMakeLib/run_compile_commands.cxx

@@ -63,12 +63,12 @@ private:
 
   void ParseString()
   {
-    this->String.clear();
+    this->String = "";
     if(!Expect('"')) return;
     while (!Expect('"'))
       {
       Expect('\\');
-      this->String.push_back(C);
+      this->String.append(1,C);
       Next();
       }
   }