type inference errors improvement

if constraint system without expected type was successful,
expected type mismatch error should be generated (instead of conflicting substitutions error)
 #KT-731 fixed

compiler/testData/diagnostics/tests/inference/regressions/kt731.kt

@@ -0,0 +1,15 @@
+package a
+
+class A<T>(x: T) {
+    val p = x
+}
+
+fun <T, G> A<T>.foo(x: (T)-> G): G {
+    return x(this.p)
+}
+
+fun main(args: Array<String>) {
+    val a = A(1)
+    val t: String = a.<!TYPE_INFERENCE_EXPECTED_TYPE_MISMATCH!>foo<!>({p -> p})
+    t : String
+}