Add back test case

Author: aschwaighofer

test/IRGen/conditional_conformance_typemetadata.swift

@@ -0,0 +1,16 @@
+// RUN: %target-swift-frontend -O -emit-ir %s
+// RUN: %target-swift-frontend -O -primary-file %s -emit-ir
+
+// Make sure that we don't crash in IRGen because the only reference to the type
+// E is in the protocol conformance descriptor describing the conditional
+// conformace.
+
+private enum E {}
+
+private protocol P { associatedtype AT }
+
+private struct S<A, B> {}
+
+extension S: P where A == E { typealias AT = B }
+
+print(S<E, Bool>.AT.self)