Disallow extension properties with backing fields

#KT-1682 Fixed

idea/testData/checker/ExtensionFunctions.kt

@@ -1,5 +1,5 @@
 fun Int?.optint() : Unit {}
-val Int?.optval : Unit = Unit.VALUE
+val Int?.optval : Unit get() = Unit.VALUE
 
 fun <T: Any, E> T.foo(<warning>x</warning> : E, y : A) : T   {
   y.plus(1)

idea/testData/checker/Redeclaration.kt

@@ -25,4 +25,4 @@ object B {
 fun PairParam<<error>T</error>, <error>T</error>>() {}
 class PParam<<error>T</error>, <error>T</error>> {}
 
-val <<error>T</error>, <error>T</error>> T.fooParam : Int = 1
+val <<error>T</error>, <error>T</error>> T.fooParam : Int get() = 1