GSB: Add regression test for already-fixed crasher

Author: slavapestov

validation-test/compiler_crashers_2_fixed/rdar76249579.swift

@@ -0,0 +1,15 @@
+// RUN: %target-swift-frontend -emit-ir %s
+
+public class Base {}
+
+public class Derived : Base {}
+
+public protocol P {}
+
+public class C : P {}
+
+public class G<X : P, Y : Base> {}
+
+extension G where X == C, Y == Derived {
+  public func foo() {}
+}