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/when/kt9929.kt

@@ -0,0 +1,9 @@
+val test: Int = if (true) {
+    when (2) {
+        1 -> 1
+        else -> <!NULL_FOR_NONNULL_TYPE!>null<!>
+    }
+}
+else {
+    2
+}