QStringBuilderCommon: drop const from resolved() result type

The const qualification prevented toUpper() etc from
calling the rvalue overloads of the corresponding QString
functions. Since resolved() always returns a non-shared
object, the rvalue overloads can re-use the object's
capacity for storing their result, saving up to one memory
allocation per QStringBuilderCommon::to*() invocation.

Change-Id: Ica97fcd906cdd949ffe56055654578b93407e2d3
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qstringbuilder.h

@@ -76,7 +76,7 @@ struct QStringBuilderCommon
     T toLower() const { return resolved().toLower(); }
 
 protected:
-    const T resolved() const { return *static_cast<const Builder*>(this); }
+    T resolved() const { return *static_cast<const Builder*>(this); }
 };
 
 template<typename Builder, typename T>