[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