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/testsWithStdLib/smartcasts/kt10463.kt

@@ -0,0 +1,6 @@
+val test: Int = listOf<Any>().map {
+    when (it) {
+        is Int -> <!DEBUG_INFO_SMARTCAST!>it<!>
+        else -> throw AssertionError()
+    }
+}.sum()

compiler/testData/diagnostics/testsWithStdLib/smartcasts/kt10463.txt

@@ -0,0 +1,3 @@
+package
+
+public val test: kotlin.Int