[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.

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() ) {