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

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

@@ -0,0 +1,33 @@
+enum class SomeEnum {
+    A, B
+}
+
+fun test_1(enum: SomeEnum) {
+    val x = <!NO_ELSE_IN_WHEN!>when<!> (enum) {
+        SomeEnum.A -> 1
+    }
+
+    val y = when (enum) {
+        SomeEnum.A -> 1
+        SomeEnum.B -> 2
+    }
+}
+
+fun test_2(enum: SomeEnum?) {
+    val x = <!NO_ELSE_IN_WHEN!>when<!> (enum) {
+        SomeEnum.A -> 1
+        SomeEnum.B -> 2
+    }
+
+    val y = when (enum) {
+        SomeEnum.A -> 1
+        SomeEnum.B -> 2
+        null -> 3
+    }
+}
+
+fun test_3(enum: SomeEnum) {
+    when (enum) {
+        SomeEnum.A -> 1
+    }
+}