K2: Resolve non-exhaustive when expressions independently

^KT-55169 Fixed
^KT-55175 Related

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

@@ -0,0 +1,13 @@
+// SKIP_TXT
+fun bar(a: String): String {
+    return <!RETURN_TYPE_MISMATCH!>when {
+        a.length == 1 -> {
+            when { // Error in K1, no error in K2
+                a == "a" -> ""
+                a == "b" -> ""
+            }
+        }
+
+        else -> ""
+    }<!>
+}

compiler/testData/diagnostics/tests/when/nonExhaustiveDependentContext.kt

@@ -0,0 +1,13 @@
+// SKIP_TXT
+fun bar(a: String): String {
+    return when {
+        a.length == 1 -> {
+            <!NO_ELSE_IN_WHEN!>when<!> { // Error in K1, no error in K2
+                a == "a" -> ""
+                a == "b" -> ""
+            }
+        }
+
+        else -> ""
+    }
+}