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/kt9972.kt

@@ -0,0 +1,17 @@
+fun test1(): Int {
+    val x: String = if (true) {
+        when {
+            true -> <!TYPE_MISMATCH!>Any()<!>
+            else -> <!NULL_FOR_NONNULL_TYPE!>null<!>
+        }
+    } else ""
+    return x.hashCode()
+}
+
+fun test2(): Int {
+    val x: String = when {
+                        true -> <!TYPE_MISMATCH!>Any()<!>
+                        else -> null
+                    } ?: return 0
+    return x.hashCode()
+}