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

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

@@ -0,0 +1,18 @@
+/*
+ * KOTLIN DIAGNOSTICS SPEC TEST (NEGATIVE)
+ *
+ * SPEC VERSION: 0.1-313
+ * PRIMARY LINKS: expressions, when-expression -> paragraph 5 -> sentence 1
+ * expressions, when-expression, exhaustive-when-expressions -> paragraph 2 -> sentence 3
+ * expressions, when-expression, exhaustive-when-expressions -> paragraph 2 -> sentence 4
+ * expressions, when-expression, exhaustive-when-expressions -> paragraph 2 -> sentence 5
+ */
+
+// See also: KT-3743
+fun foo(arg: Boolean): String {
+    // Must be exhaustive
+    return <!NO_ELSE_IN_WHEN!>when<!>(arg) {
+        2 == 2 -> "truth"
+        2 == 1 -> "falsehood"
+    }
+}