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/psi/recovery/DeclarationAfterIncompleteElvis.kt

@@ -0,0 +1,21 @@
+fun foo(x: Any?) {
+    x ?:
+    val foo = 1
+
+    x ?:
+    fun bar() = 2
+
+    x ?:
+    fun String.() = 3
+}
+
+class A {
+    val z = null ?:
+    val x = 4
+
+    val y = null ?:
+    fun baz() = 5
+
+    val q = null ?:
+    fun String.() = 6
+}