QString: make insert_helper take the insertee by const-ref

Not all callers pass a view-type object: there are several call points
passing a QVarLengthArray, which means the value ended up being copied.

Introduced in 67108ef8db. Found by
Coverity scan (CID 404702).

Change-Id: I9671dee8ceb64aa9b9cafffd1742f9cda2131752
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/text/qstring.cpp

@@ -2943,7 +2943,7 @@ QString &QString::operator=(QChar ch)
     T is a view or a container on/of QChar, char16_t, or char
 */
 template <typename T>
-static void insert_helper(QString &str, qsizetype i, T toInsert)
+static void insert_helper(QString &str, qsizetype i, const T &toInsert)
 {
     auto &str_d = str.data_ptr();
     qsizetype difference = 0;