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

compiler/testData/diagnostics/tests/unit/nullableUnit.fir.kt

@@ -1,5 +0,0 @@
-fun foo() {}
-
-val x: Unit? = when ("A") {
-    "B" -> foo()
-}

compiler/testData/diagnostics/tests/unit/nullableUnit.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 fun foo() {}
 
 val x: Unit? = <!NO_ELSE_IN_WHEN!>when<!> ("A") {