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

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

@@ -0,0 +1,33 @@
+fun test_1(cond: Boolean) {
+    val x = <!NO_ELSE_IN_WHEN!>when<!> (cond) {
+        true -> 1
+    }
+
+    val y = <!NO_ELSE_IN_WHEN!>when<!> (cond) {
+        false -> 2
+    }
+
+    val z = when (cond) {
+        true -> 1
+        false -> 2
+    }
+}
+
+fun test_2(cond: Boolean?) {
+    val x = <!NO_ELSE_IN_WHEN!>when<!> (cond) {
+        true -> 1
+        false -> 2
+    }
+
+    val x = when (cond) {
+        true -> 1
+        false -> 2
+        null -> 3
+    }
+}
+
+fun test_3(cond: Boolean) {
+    when (cond) {
+        true -> 1
+    }
+}