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.txt

@@ -0,0 +1,8 @@
+package
+
+public fun </*0*/ T> bar(/*0*/ x: T, /*1*/ y: T): kotlin.Unit
+public fun foo(): kotlin.Unit
+public fun test1(): kotlin.Unit
+public fun test2(): kotlin.Unit
+public fun test3(): kotlin.Unit
+public fun test4(): kotlin.Unit