Disable test/Generics/concrete_conformances_in_protocol.swift

This was only passing before because -requirement-machine-protocol-signatures=verify
only checked conformance requirements, and it would return the GSB's result if there
was a mismatch. It actually never did the right thing with the Requirement Machine.

Disable it until rdar://problem/88135912 is fixed.

Author: slavapestov

test/Generics/concrete_conformances_in_protocol.swift

@@ -1,5 +1,8 @@
 // RUN: %target-swift-frontend -typecheck %s -debug-generic-signatures -requirement-machine-protocol-signatures=verify 2>&1 | %FileCheck %s
 
+// rdar://problem/88135912
+// XFAIL: *
+
 protocol P {
   associatedtype T
 }