[FIR] Implement checker for exhaustive when's in expression position

compiler/fir/analysis-tests/testData/resolve/exhaustiveness/negative/missingElse.kt

@@ -0,0 +1,23 @@
+fun test(a: Any) {
+    val x = <!NO_ELSE_IN_WHEN!>when<!> (a) {
+        is Int -> 1
+        is String -> 2
+    }
+
+    val y = when (a) {
+        else -> 1
+    }
+
+    val z = when (a) {
+        is Int -> 1
+        is String -> 2
+        else -> 3
+    }
+}
+
+fun test_2(a: Any) {
+    when (a) {
+        is String -> 1
+        is Int -> 2
+    }
+}