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/kt10439.kt

@@ -0,0 +1,12 @@
+fun foo(x: Int) = x
+
+fun test0(flag: Boolean) {
+    foo(if (flag) <!CONSTANT_EXPECTED_TYPE_MISMATCH!>true<!> else <!TYPE_MISMATCH!>""<!>)
+}
+
+fun test1(flag: Boolean) {
+    foo(when (flag) {
+        true -> <!CONSTANT_EXPECTED_TYPE_MISMATCH!>true<!>
+        else -> <!TYPE_MISMATCH!>""<!>
+    })
+}