PatternMatchingTypingVisitor:

rewrite type inference for 'when' using special constructs.
This fixes several type inference issues for 'when':
KT-9929, KT-9972, KT-10439, KT-10463
along with some other diagnostics-related issues.

compiler/testData/diagnostics/tests/smartCasts/whenExprNonNull.kt

@@ -3,16 +3,16 @@ fun baz(s: String?): String {
     // if explicit type String is given for t, problem disappears
     val t = when(<!DEBUG_INFO_SMARTCAST!>s<!>) {
         // !! is detected as unnecessary here
-        "abc" -> s
+        "abc" -> <!DEBUG_INFO_SMARTCAST!>s<!>
         else -> "xyz"
     }
-    return <!DEBUG_INFO_SMARTCAST!>t<!>
+    return t
 }
 
 fun foo(s: String?): String {
     val t = when {
-        s != null -> s
+        s != null -> <!DEBUG_INFO_SMARTCAST!>s<!>
         else -> ""
     }
-    return <!DEBUG_INFO_SMARTCAST!>t<!>
+    return t
 }