Allow inferring property type from its getter

#KT-550 Fixed

compiler/testData/diagnostics/tests/properties/inferenceFromGetters/nullAsNothing.kt

@@ -0,0 +1,8 @@
+// !CHECK_TYPE
+val x get() = null
+val <!IMPLICIT_NOTHING_PROPERTY_TYPE!>y<!> get() = null!!
+
+fun foo() {
+    <!DEBUG_INFO_CONSTANT!>x<!> checkType { _<Nothing?>() }
+    y <!UNREACHABLE_CODE!>checkType { _<Nothing>() }<!>
+}