Template metadata structures

- Add RuntimeTarget template This will allow for converting between
  metadata structures for native host and remote target architectures.

- Create InProcess and External templates for stored pointers

Add a few more types to abstract pointer access in the runtime
structures but keep native in-process pointer access the same as that
with a plain old pointer type.

There is now a notion of a "stored pointer", which is just the raw value
of the pointer, and the actual pointer type, which is used for loads.
Decoupling these allows us to fork the behavior when looking at metadata
in an external process, but keep things the same for the in-process
case.

There are two basic "runtime targets" that you can use to work with
metadata:

InProcess: Defines the pointer to be trivially a T* and stored as a
uintptr_t. A Metadata * is exactly as it was before, but defined via
AbstractMetadata<InProcess>.

External: A template that requires a target to specify its pointer size.

ExternalPointer: An opaque pointer in another address space that can't
(and shouldn't) be indirected with operator* or operator->.  The memory
reader will fetch the data explicitly.

stdlib/public/runtime/MetadataLookup.cpp

@@ -73,10 +73,10 @@ namespace {
 
   public:
     TypeMetadataCacheEntry(const llvm::StringRef name,
-                           const struct Metadata *metadata)
+                           const ::Metadata *metadata)
       : Name(name.str()), Metadata(metadata) {}
 
-    const struct Metadata *getMetadata(void) {
+    const ::Metadata *getMetadata(void) {
       return Metadata;
     }
 
@@ -230,6 +230,7 @@ swift::swift_registerTypeMetadataRecords(const TypeMetadataRecord *begin,
 }
 
 // copied from ProtocolConformanceRecord::getCanonicalTypeMetadata()
+template<>
 const Metadata *TypeMetadataRecord::getCanonicalTypeMetadata() const {
   switch (getTypeKind()) {
   case TypeMetadataRecordKind::UniqueDirectType:
@@ -238,7 +239,7 @@ const Metadata *TypeMetadataRecord::getCanonicalTypeMetadata() const {
     return swift_getForeignTypeMetadata((ForeignTypeMetadata *)getDirectType());
   case TypeMetadataRecordKind::UniqueDirectClass:
     if (auto *ClassMetadata =
-          static_cast<const struct ClassMetadata *>(getDirectType()))
+          static_cast<const ::ClassMetadata *>(getDirectType()))
       return swift_getObjCClassMetadata(ClassMetadata);
     else
       return nullptr;