Support for new double colon expressions in type checker

Despite the fact that the LHS of a double colon expression is now KtExpression
instead of KtUserType, we must still consider it a type for callable references
to work. Extract and reuse relevant parts of QualifiedExpressionResolver and
TypeResolver to support this behavior

compiler/testData/diagnostics/tests/callableReference/whitespacesInExpression.kt

@@ -0,0 +1,16 @@
+// !CHECK_TYPE
+
+class Foo
+
+fun Foo?.bar() {}
+
+fun test() {
+    val r1 = Foo ?:: bar
+    checkSubtype<(Foo?) -> Unit>(r1)
+
+    val r2 = Foo ? :: bar
+    checkSubtype<(Foo?) -> Unit>(r2)
+
+    val r3 = Foo ? ? :: bar
+    checkSubtype<(Foo?) -> Unit>(r3)
+}