Disallow extension properties with backing fields

#KT-1682 Fixed

compiler/testData/diagnostics/tests/resolve/invoke/invokeAsExtension.kt

@@ -24,7 +24,8 @@ fun test(a: A) {
 //variable as extension
 trait B {
 }
-val B.foo = Foo()
+val B.foo: Foo
+    get() = Foo()
 
 fun test(b: B) {
     b.foo()

compiler/testData/diagnostics/tests/resolve/invoke/invokeAsMember.kt

@@ -22,7 +22,8 @@ fun test(a: A) {
 
 //variable as extension
 trait B {}
-val B.foo = Foo()
+val B.foo: Foo
+    get() = Foo()
 
 
 fun test(b: B) {

compiler/testData/diagnostics/tests/resolve/invoke/invokeAsMemberExtension.kt

@@ -40,7 +40,8 @@ fun test(c: C, b: B) {
 //variable as extension,
 trait D {
 }
-val D.foo = Foo()
+val D.foo: Foo
+    get() = Foo()
 
 class E {
     fun Foo.invoke() {}

compiler/testData/diagnostics/tests/resolve/invoke/kt3772.kt

@@ -10,10 +10,12 @@ class B {
 }
 
 open class C
-val C.attr = A()
+val C.attr: A
+    get() = A()
 
 open class D: C()
-val D.attr = B()
+val D.attr: B
+    get() = B()
 
 
 fun main(args: Array<String>) {