Introduce separate callee expression for when with subject variable

'Subject.Error' is redundant.
'Subject.None' can be an object.
'Subject#dataFlowValue' can be a lateinit property.

TODO: fix
- parsing local extension properties in 'when' subject
- parsing destructuring declarations in 'when' subject
- non-completed calls in nested 'when' with subject variable
- non-completed calls for subject variable in 'in' pattern

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

@@ -2,10 +2,26 @@
 // !DIAGNOSTICS: -UNUSED_VARIABLE -UNUSED_PARAMETER
 
 fun foo(): Any = 42
+fun useInt(i: Int) {}
 
-fun test(y: Any) {
-    val z = when (val <!NAME_SHADOWING!>y<!> = foo()) {
-        42 -> "Magic: $y"
-        else -> "Not magic: $y"
+fun testShadowingParameter(y: Any) {
+    when (val <!NAME_SHADOWING!>y<!> = foo()) {
+        else -> {}
+    }
+}
+
+fun testShadowedInWhenBody(x: Any) {
+    when (val y = x) {
+        is String -> {
+            val <!NAME_SHADOWING!>y<!> = <!DEBUG_INFO_SMARTCAST!>y<!>.length
+            useInt(y)
+        }
+    }
+}
+
+fun testShadowinLocalVariable() {
+    val y = foo()
+    when (val <!NAME_SHADOWING!>y<!> = foo()) {
+        else -> {}
     }
 }