Use constraint system for comparing for specificity against a generic signature.

Drop "discrimiate generics" mode where it's unneeded.

compiler/testData/diagnostics/tests/resolve/overloadConflicts/varargWithMoreSpecificSignature.kt

@@ -0,0 +1,9 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+object Foo1
+object Foo2
+
+fun foo(vararg ss: String) = Foo1
+fun foo(x: Any) = Foo2
+
+val test1: Foo1 = foo("")