[Demangler][Tests] Address Doug's comments.

Moved the test for the metadata kind to MetadataLookup.cpp.

Added an assertion (for debug builds) to Metadata.cpp to catch the case where
something manages to bypass that test.

Author: al45tair

stdlib/public/runtime/Metadata.cpp

@@ -3484,22 +3484,23 @@ ExistentialMetatypeValueWitnessTableCacheEntry(unsigned numWitnessTables) {
 SWIFT_RUNTIME_EXPORT
 const ExistentialMetatypeMetadata *
 swift::swift_getExistentialMetatypeMetadata(const Metadata *instanceMetadata) {
-  if (instanceMetadata->getKind() != MetadataKind::Existential
-      && instanceMetadata->getKind() != MetadataKind::ExistentialMetatype)
-    return nullptr;
-
   return &ExistentialMetatypes.getOrInsert(instanceMetadata).first->Data;
 }
 
 ExistentialMetatypeCacheEntry::ExistentialMetatypeCacheEntry(
                                             const Metadata *instanceMetadata) {
   ExistentialTypeFlags flags;
-  if (instanceMetadata->getKind() == MetadataKind::Existential) {
+  switch (instanceMetadata->getKind()) {
+  case MetadataKind::Existential:
     flags = static_cast<const ExistentialTypeMetadata*>(instanceMetadata)
       ->Flags;
-  } else {
+    break;
+  case MetadataKind::ExistentialMetatype:
     flags = static_cast<const ExistentialMetatypeMetadata*>(instanceMetadata)
       ->Flags;
+    break;
+  default:
+    assert(false && "expected existential metadata");
   }
 
   Data.setKind(MetadataKind::ExistentialMetatype);

stdlib/public/runtime/MetadataLookup.cpp

@@ -1446,13 +1446,13 @@ class DecodedMetadataBuilder {
   TypeLookupErrorOr<BuiltType> createExistentialMetatypeType(
       BuiltType instance,
       llvm::Optional<Demangle::ImplMetatypeRepresentation> repr = None) const {
-    BuiltType result = swift_getExistentialMetatypeMetadata(instance);
-    if (!result) {
+    if (instance->getKind() != MetadataKind::Existential
+        && instance->getKind() != MetadataKind::ExistentialMetatype) {
       return TYPE_LOOKUP_ERROR_FMT("Tried to build an existential metatype from "
                                    "a type that was neither an existential nor "
                                    "an existential metatype");
     }
-    return result;
+    return swift_getExistentialMetatypeMetadata(instance);
   }
 
   TypeLookupErrorOr<BuiltType>