Handle nested when with subject properly

Hack: callee expression for when with subject variable is the subject
variable declaration. This solves the problem that all sub-calls in the
expression are implicitly considered to have a single common lexical
scope (and 'when (val x = ...)' introduces a new lexical scope, which
contains 'x').

compiler/testData/diagnostics/tests/when/withSubjectVariable/nestedWhenWithSubject.kt

@@ -0,0 +1,41 @@
+// !LANGUAGE: +VariableDeclarationInWhenSubject
+// !DIAGNOSTICS: -UNUSED_VARIABLE -UNUSED_PARAMETER -UNUSED_VALUE
+
+fun foo() {}
+fun <T> bar(x: T, y: T) {}
+
+fun test1() {
+    when (1) {
+        1 ->
+            when (val y = 2) {
+                2 -> foo()
+            }
+    }
+}
+
+fun test2() {
+    when (val x = 1) {
+        1 ->
+            when (val y = 2) {
+                2 -> foo()
+            }
+    }
+}
+
+fun test3() {
+    when (val x = 1) {
+        1 ->
+            when (val <!NAME_SHADOWING!>x<!> = 2) {
+                2 -> foo()
+            }
+    }
+}
+
+fun test4() {
+    when (val x = 1) {
+        1 ->
+            when (val y = 2) {
+                2 -> bar(x, y)
+            }
+    }
+}