use data flow info in constraint system

intersect data flow value possible types
add intersection type (or exact type for the most cases) as a lower bound to constraint system

always get common super type for intersection type as a result
(avoid returning it, even in error messages)

compiler/testData/diagnostics/tests/smartCasts/inference/kt2851.kt

@@ -0,0 +1,20 @@
+//KT-2851 Type inference failed passing in not-null after smart-cast value in Pair
+package a
+
+fun main(args: Array<String>) {
+    val value: String? = ""
+    if (value != null) {
+        foo(Pair("val", value))
+        foo(Pair("val", value<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>))
+        foo(Pair<String, String>("val", value))
+    }
+}
+
+fun foo(<!UNUSED_PARAMETER!>map<!>: Pair<String, String>) {}
+
+
+//from library
+public class Pair<out A, out B> (
+        public val first: A,
+        public val second: B
+)