FIR: Support exhaustive whens on subjects of intersection type

compiler/testData/diagnostics/tests/when/intersectionExhaustivenessComplex.fir.kt

@@ -0,0 +1,42 @@
+// !CHECK_TYPE
+
+sealed class A {
+    class A1 : A()
+    class A2 : A()
+}
+
+sealed class B {
+    class B1 : B()
+    class B2 : B()
+}
+
+fun foo(a: A) {
+    if (a !is B) return
+
+    when (a) {
+        is A.A1 -> ""
+        is A.A2 -> "v"
+    }.length
+
+    when (a) {
+        is A.A1 -> ""
+        is A.A2 -> "v"
+    }.length // OK
+
+    when (a) {
+        is A.A1 -> ""
+        is A.A2 -> "v"
+        is B.B1 -> "..." // should be warning: unreachable code
+    }.length // OK
+
+    when (a) {
+        is A.A1 -> ""
+        is B.B1 -> "..."
+        is A.A2 -> "v"
+    }.length // OK
+
+    <!NO_ELSE_IN_WHEN!>when<!> (a) {
+        is A.A1 -> ""
+        is B.B1 -> "..."
+    }.<!UNRESOLVED_REFERENCE!>length<!>
+}