qFindChar: pass char16_t by value

Cheaply-copied types should be passed by value
according to https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rf-in

Change-Id: I341193c0a65931c1406fc7d711ef74d763178dee
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/text/qstring.cpp

@@ -150,7 +150,7 @@ static inline qsizetype qFindChar(QStringView str, QChar ch, qsizetype from, Qt:
                 return n - s;
         } else {
             c = foldCase(c);
-            auto it = std::find_if(n, e, [c](const auto &ch) { return foldAndCompare(ch, c); });
+            auto it = std::find_if(n, e, [c](auto ch) { return foldAndCompare(ch, c); });
             if (it != e)
                 return std::distance(s, it);
         }