Optimize iteration over CharSequence's on JVM

KT-7931 Optimize iteration over strings/charsequences on JVM

compiler/testData/codegen/box/controlStructures/forInCharSequence.kt

@@ -0,0 +1,9 @@
+// WITH_RUNTIME
+
+fun box(): String {
+    var s = ""
+    for (c in StringBuilder("OK")) {
+        s += c
+    }
+    return s
+}