Implementation and tests for KT-5840 and newly created KT-7204. Now a safe call provides not-null receiver state *inside* argument list. It works also for ?. chains. #KT-5840 Fixed.

On the other hand, argument states do not propagate to successor statements for a safe call. #KT-7204 Fixed. A few additional comments.

compiler/testData/diagnostics/tests/smartCasts/safecalls/chainAndUse.kt

@@ -0,0 +1,6 @@
+fun calc(x: List<String>?): Int {
+    // x should be non-null in arguments list, despite of a chain
+    x?.subList(0, 1)?.get(<!DEBUG_INFO_SMARTCAST!>x<!>.size())
+    // But not here!
+    return x!!.size()
+}