[AST] Use a proper “empty key” encoding for CanType in DenseMaps.

… rather than co-opting NULL, which is a valid CanType type.

Author: DougGregor

include/swift/AST/Type.h

@@ -647,7 +647,8 @@ namespace llvm {
   template<> struct DenseMapInfo<swift::CanType>
     : public DenseMapInfo<swift::Type> {
     static swift::CanType getEmptyKey() {
-      return swift::CanType(nullptr);
+      return swift::CanType(llvm::DenseMapInfo<swift::
+                              TypeBase*>::getEmptyKey());
     }
     static swift::CanType getTombstoneKey() {
       return swift::CanType(llvm::DenseMapInfo<swift::

include/swift/AST/Types.h

@@ -5346,6 +5346,7 @@ inline CanType CanType::getNominalParent() const {
 
 inline bool CanType::isActuallyCanonicalOrNull() const {
   return getPointer() == nullptr ||
+         getPointer() == llvm::DenseMapInfo<TypeBase *>::getEmptyKey() ||
          getPointer() == llvm::DenseMapInfo<TypeBase *>::getTombstoneKey() ||
          getPointer()->isCanonical();
 }