[metadata prespecialization] Target only >=5.2.

Compatibility with earlier swift runtimes would require modifying the
runtime compatibility libraries to adjust the behavior of
checkMetadataState by way of typeForMangledNode or even
typeForMangledName.  For now, simply require that a version of swift
whose runtime knows about prespecialized metadata is being targeted.

include/swift/AST/ASTContext.h

@@ -614,6 +614,14 @@ public:
   /// swift_getTypeByMangledNameInContextInMetadataState.
   AvailabilityContext getTypesInAbstractMetadataStateAvailability();
 
+  /// Get the runtime availability of support for prespecialized generic 
+  /// metadata.
+  AvailabilityContext getPrespecializedGenericMetadataAvailability();
+
+  /// Get the runtime availability of features introduced in the Swift 5.2
+  /// compiler for the target platform.
+  AvailabilityContext getSwift52Availability();
+
 
   //===--------------------------------------------------------------------===//
   // Diagnostics Helper functions

lib/AST/Availability.cpp

@@ -239,6 +239,14 @@ AvailabilityContext ASTContext::getSwift51Availability() {
 }
 
 AvailabilityContext ASTContext::getTypesInAbstractMetadataStateAvailability() {
+  return getSwift52Availability();
+}
+
+AvailabilityContext ASTContext::getPrespecializedGenericMetadataAvailability() {
+  return getSwift52Availability();
+}
+
+AvailabilityContext ASTContext::getSwift52Availability() {
   auto target = LangOpts.Target;
 
   if (target.isMacOSX() ) {

lib/IRGen/IRGenModule.cpp

@@ -1329,6 +1329,15 @@ void IRGenModule::error(SourceLoc loc, const Twine &message) {
 
 bool IRGenModule::useDllStorage() { return ::useDllStorage(Triple); }
 
+bool IRGenModule::shouldPrespecializeGenericMetadata() {
+  auto &context = getSwiftModule()->getASTContext();
+  auto deploymentAvailability =
+      AvailabilityContext::forDeploymentTarget(context);
+  return IRGen.Opts.PrespecializeGenericMetadata && 
+    deploymentAvailability.isContainedIn(
+      context.getPrespecializedGenericMetadataAvailability());
+}
+
 void IRGenerator::addGenModule(SourceFile *SF, IRGenModule *IGM) {
   assert(GenModules.count(SF) == 0);
   GenModules[SF] = IGM;

lib/IRGen/IRGenModule.h

@@ -744,6 +744,8 @@ public:
   void error(SourceLoc loc, const Twine &message);
 
   bool useDllStorage();
+
+  bool shouldPrespecializeGenericMetadata();
   
   Size getAtomicBoolSize() const { return AtomicBoolSize; }
   Alignment getAtomicBoolAlignment() const { return AtomicBoolAlign; }