Do not lose information about contracts if call uses default value

It is safe to treat DefaultValueArgument as UNKNOWN_COMPUTATION, because
default arguments can't break smartcasts.

Possibly, they can add new ones, but it can be supported later.

^KT-25278 Fixed

compiler/testData/diagnostics/testsWithStdLib/contracts/smartcasts/callWithDefaultValue.txt

@@ -0,0 +1,6 @@
+package
+
+public fun myAssert(/*0*/ condition: kotlin.Boolean, /*1*/ message: kotlin.String = ...): kotlin.Unit
+    Returns(WILDCARD) -> condition
+
+public fun test(/*0*/ x: kotlin.Any?): kotlin.Unit