Provide implicit receiver to extension property initializer

This is safe because initializers for extension properties are forbidden
anyway, we'll just won't be generating noisy error messages about non-existent
'this'

compiler/testData/diagnostics/tests/scopes/initializerScopeOfExtensionProperty.kt

@@ -1,10 +1,10 @@
 package i
 
-val <T> List<T>.length = <!EXTENSION_PROPERTY_WITH_BACKING_FIELD!><!UNRESOLVED_REFERENCE!>size<!>()<!>
+val <T> List<T>.length = <!EXTENSION_PROPERTY_WITH_BACKING_FIELD!>size()<!>
 
 val <T> List<T>.length1 : Int get() = size()
 
-val String.bd = <!EXTENSION_PROPERTY_WITH_BACKING_FIELD!><!NO_THIS!>this<!> <!DEBUG_INFO_ELEMENT_WITH_ERROR_TYPE!>+<!> "!"<!>
+val String.bd = <!EXTENSION_PROPERTY_WITH_BACKING_FIELD!>this + "!"<!>
 
 val String.bd1 : String get() = this + "!"
 
@@ -13,7 +13,7 @@ class A {
     val ii : Int = 1
 }
 
-val A.foo = <!UNRESOLVED_REFERENCE, EXTENSION_PROPERTY_WITH_BACKING_FIELD!>ii<!>
+val A.foo = <!EXTENSION_PROPERTY_WITH_BACKING_FIELD!>ii<!>
 
 val A.foo1 : Int get() = ii