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/subjectVariableInIsPattern.kt

@@ -0,0 +1,8 @@
+// !LANGUAGE: +VariableDeclarationInWhenSubject
+// !DIAGNOSTICS: -UNUSED_VARIABLE -UNUSED_PARAMETER
+
+fun test(x: Any) {
+    when (val y = x) {
+        is String -> {}
+    }
+}