Implement recovery for incomplete expression before declaration

It's needed when declarations are parsed as a part of previous expression
(see tests)

Currently we apply this kind of recovery in a conservative way,
only when declaration starts at the next line, and while
the condition could be relaxed, there's no need to do this

 #KT-4948 Fixed
 #KT-7118 Fixed

compiler/testData/diagnostics/tests/incompleteCode/variableDeclarationInSelector.kt

@@ -1,4 +1,4 @@
 fun foo(s: String) {
-    s.
-    <!DECLARATION_IN_ILLEGAL_CONTEXT, ILLEGAL_SELECTOR!>val b = 42<!>
-}
+    s.<!SYNTAX!><!>
+    val <!UNUSED_VARIABLE!>b<!> = 42
+}