[Runtime][IRGen] Sign type context descriptor pointers.

Ensure that context descriptor pointers are signed in the runtime by putting the ptrauth_struct attribute on the types.

We use the new __builtin_ptrauth_struct_key/disc to conditionally apply ptrauth_struct to TrailingObjects based on the signing of the base type, so that pointers to TrailingObjects get signed when used with a context descriptor pointer.

We add new runtime entrypoints that take signed pointers where appropriate, and have the compiler emit calls to the new entrypoints when targeting a sufficiently new OS.

rdar://111480914

stdlib/public/runtime/ProtocolConformance.cpp

@@ -1147,8 +1147,8 @@ swift_conformsToProtocolMaybeInstantiateSuperclasses(
 }
 
 static const WitnessTable *
-swift_conformsToProtocolImpl(const Metadata *const type,
-                             const ProtocolDescriptor *protocol) {
+swift_conformsToProtocolCommonImpl(const Metadata *const type,
+                                   const ProtocolDescriptor *protocol) {
   const WitnessTable *table;
   bool hasUninstantiatedSuperclass;
 
@@ -1173,6 +1173,26 @@ swift_conformsToProtocolImpl(const Metadata *const type,
   return table;
 }
 
+static const WitnessTable *
+swift_conformsToProtocol2Impl(const Metadata *const type,
+                              const ProtocolDescriptor *protocol) {
+  protocol = swift_auth_data_non_address(
+      protocol, SpecialPointerAuthDiscriminators::ProtocolDescriptor);
+  return swift_conformsToProtocolCommonImpl(type, protocol);
+}
+
+static const WitnessTable *
+swift_conformsToProtocolImpl(const Metadata *const type,
+                             const void *protocol) {
+  // This call takes `protocol` without a ptrauth signature. We declare
+  // it as `void *` to avoid the implicit ptrauth we get from the
+  // ptrauth_struct attribute. The static_cast implicitly signs the
+  // pointer when we call through to the implementation in
+  // swift_conformsToProtocolCommon.
+  return swift_conformsToProtocolCommonImpl(
+      type, static_cast<const ProtocolDescriptor *>(protocol));
+}
+
 const ContextDescriptor *
 swift::_searchConformancesByMangledTypeName(Demangle::NodePointer node) {
   auto traceState = runtime::trace::protocol_conformance_scan_begin(node);