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/withVariance.kt

@@ -0,0 +1,9 @@
+object X1
+object X2
+
+class A<T>
+
+fun <T1> A<T1>.foo() = X1
+fun <T2> A<out T2>.foo() = X2
+
+fun <T> A<out T>.test() = <!CANNOT_COMPLETE_RESOLVE!>foo<!>()