[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

lib/IRGen/IRGenModule.cpp

@@ -888,6 +888,24 @@ namespace RuntimeConstants {
     return RuntimeAvailability::AlwaysAvailable;
   }
 
+  RuntimeAvailability SignedConformsToProtocolAvailability(ASTContext &context) {
+    auto featureAvailability =
+        context.getSignedConformsToProtocolAvailability();
+    if (!isDeploymentAvailabilityContainedIn(context, featureAvailability)) {
+      return RuntimeAvailability::ConditionallyAvailable;
+    }
+    return RuntimeAvailability::AlwaysAvailable;
+  }
+
+  RuntimeAvailability SignedDescriptorAvailability(ASTContext &context) {
+    auto featureAvailability =
+        context.getSignedDescriptorAvailability();
+    if (!isDeploymentAvailabilityContainedIn(context, featureAvailability)) {
+      return RuntimeAvailability::ConditionallyAvailable;
+    }
+    return RuntimeAvailability::AlwaysAvailable;
+  }
+
   RuntimeAvailability TaskRunInlineAvailability(ASTContext &context) {
     if (context.LangOpts.isConcurrencyModelTaskToThread()) {
       return RuntimeAvailability::AlwaysAvailable;