[ABI] Fix metadata accessors for 32-bit remote AST/remote mirrors.

The use of sizeof(void*) in TargetStructMetadata and
TargetEnumMetadata's accessors is incorrect when (e.g.) reading
metadata from a 32-bit process in a 64-bit host. Use
sizeof(StoredPointer) instead to properly account for the runtime
pointer size.

Fixes rdar://problem/47305557.

Author: DougGregor

include/swift/ABI/Metadata.h

@@ -1321,7 +1321,7 @@ struct TargetStructMetadata : public TargetValueMetadata<Runtime> {
   }
 
   static constexpr int32_t getGenericArgumentOffset() {
-    return sizeof(TargetStructMetadata<Runtime>) / sizeof(void*);
+    return sizeof(TargetStructMetadata<Runtime>) / sizeof(StoredPointer);
   }
 
   static bool classof(const TargetMetadata<Runtime> *metadata) {
@@ -1369,7 +1369,7 @@ struct TargetEnumMetadata : public TargetValueMetadata<Runtime> {
   }
 
   static constexpr int32_t getGenericArgumentOffset() {
-    return sizeof(TargetEnumMetadata<Runtime>) / sizeof(void*);
+    return sizeof(TargetEnumMetadata<Runtime>) / sizeof(StoredPointer);
   }
 
   static bool classof(const TargetMetadata<Runtime> *metadata) {
@@ -3537,11 +3537,11 @@ class TargetTypeContextDescriptor
   llvm::ArrayRef<GenericParamDescriptor> getGenericParams() const;
 
   /// Return the offset of the start of generic arguments in the nominal
-  /// type's metadata. The returned value is measured in sizeof(void*).
+  /// type's metadata. The returned value is measured in sizeof(StoredPointer).
   int32_t getGenericArgumentOffset() const;
 
   /// Return the start of the generic arguments array in the nominal
-  /// type's metadata. The returned value is measured in sizeof(void*).
+  /// type's metadata. The returned value is measured in sizeof(StoredPointer).
   const TargetMetadata<Runtime> * const *getGenericArguments(
                                const TargetMetadata<Runtime> *metadata) const {
     auto offset = getGenericArgumentOffset();