QString: optimize remove(QChar, Qt::CaseSensitivity)

remove(int, int) with O(N) was used in a loop.
We had a quadratic complexity.

Use erase-remove idiom to fix it.

Change-Id: I643a2a75619ec5ea2bf99e48a25f64a7f69ba156
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/corelib/tools/qstring.cpp

@@ -2300,21 +2300,20 @@ QString &QString::remove(const QString &str, Qt::CaseSensitivity cs)
 */
 QString &QString::remove(QChar ch, Qt::CaseSensitivity cs)
 {
-    int i = 0;
+    const int idx = indexOf(ch, 0, cs);
-    ushort c = ch.unicode();
+    if (idx != -1) {
+        const auto first = begin(); // implicit detach()
+        auto last = end();
         if (cs == Qt::CaseSensitive) {
-        while (i < d->size)
+            last = std::remove(first + idx, last, ch);
-            if (d->data()[i] == ch)
-                remove(i, 1);
-            else
-                i++;
         } else {
-        c = foldCase(c);
+            const QChar c = ch.toCaseFolded();
-        while (i < d->size)
+            auto caseInsensEqual = [c](QChar x) {
-            if (foldCase(d->data()[i]) == c)
+                return c == x.toCaseFolded();
-                remove(i, 1);
+            };
-            else
+            last = std::remove_if(first + idx, last, caseInsensEqual);
-                i++;
+        }
+        resize(last - first);
     }
     return *this;
 }