qstringalgorithms: refactor trimmed_helper_positions

Take by const Str&, trimming a container/view of characters means
removing whitespace from the beginning and end, so the two args were
always cbegin() and cend().

Change-Id: Iac0eda59341f1981204d11013d591013eae3c4e0
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/text/qbytearray.cpp

@@ -3657,9 +3657,7 @@ QByteArray QByteArray::trimmed_helper(QByteArray &a)
 
 QByteArrayView QtPrivate::trimmed(QByteArrayView view) noexcept
 {
-    auto start = view.begin();
-    auto stop = view.end();
-    QStringAlgorithms<QByteArrayView>::trimmed_helper_positions(start, stop);
+    const auto [start, stop] = QStringAlgorithms<QByteArrayView>::trimmed_helper_positions(view);
     return QByteArrayView(start, stop);
 }
 

src/corelib/text/qstring.cpp

@@ -5995,9 +5995,7 @@ namespace {
     template <typename StringView>
     StringView qt_trimmed(StringView s) noexcept
     {
-        auto begin = s.begin();
-        auto end = s.end();
-        QStringAlgorithms<const StringView>::trimmed_helper_positions(begin, end);
+        const auto [begin, end] = QStringAlgorithms<const StringView>::trimmed_helper_positions(s);
         return StringView{begin, end};
     }
 }

src/corelib/text/qstringalgorithms_p.h

@@ -50,22 +50,31 @@ template <typename StringType> struct QStringAlgorithms
         Q_UNREACHABLE_RETURN(StringType());
     }
 
-    static inline void trimmed_helper_positions(const Char *&begin, const Char *&end)
+    struct TrimPositions {
+        const Char *begin;
+        const Char *end;
+    };
+    // Returns {begin, end} where:
+    // - "begin" refers to the first non-space character
+    // - if there is a sequence of one or more space chacaters at the end,
+    //   "end" refers to the first character in that sequence, otherwise
+    //   "end" is str.cend()
+    static TrimPositions trimmed_helper_positions(const StringType &str)
     {
+        const Char *begin = str.cbegin();
+        const Char *end = str.cend();
         // skip white space from end
         while (begin < end && isSpace(end[-1]))
             --end;
         // skip white space from start
         while (begin < end && isSpace(*begin))
             begin++;
+        return {begin, end};
     }
 
     static inline StringType trimmed_helper(StringType &str)
     {
-        const Char *begin = str.cbegin();
-        const Char *end = str.cend();
-        trimmed_helper_positions(begin, end);
-
+        const auto [begin, end] = trimmed_helper_positions(str);
         if (begin == str.cbegin() && end == str.cend())
             return str;
         if (!isConst && str.isDetached())