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.kt

@@ -0,0 +1,16 @@
+// !LANGUAGE: +AllowContractsForCustomFunctions +UseReturnsEffect
+// !DIAGNOSTICS: -INVISIBLE_REFERENCE -INVISIBLE_MEMBER
+
+import kotlin.internal.contracts.*
+
+fun myAssert(condition: Boolean, message: String = "") {
+    contract {
+        returns() implies (condition)
+    }
+    if (!condition) throw kotlin.IllegalArgumentException(message)
+}
+
+fun test(x: Any?) {
+    myAssert(x is String)
+    <!DEBUG_INFO_SMARTCAST!>x<!>.length
+}

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