Fix metadata accessor function pointer representation on WASI

The metadata accessor function pointer is represented as a relative
pointer on all platforms except for WebAssembly, where it is represented
as an absolute pointer. This is because WebAssembly does not support
relative function pointers.

See forum discussion for more context:
https://forums.swift.org/t/wasm-support/16087/39

Author: kateinoigakukun

Sources/_TestingInternals/Discovery.cpp

@@ -86,6 +86,26 @@ struct SWTRelativePointer {
   }
 };
 
+/// A type representing a 32-bit absolute function pointer, usually used on platforms
+/// where relative function pointers are not supported.
+///
+/// This type is derived from `AbsoluteFunctionPointer` in the Swift repository.
+template <typename T>
+struct SWTAbsoluteFunctionPointer {
+private:
+  T *_pointer;
+  static_assert(sizeof(T *) == sizeof(int32_t), "Function pointer must be 32-bit when using compact absolute pointer");
+
+public:
+  const T *_Nullable get(void) const & {
+    return _pointer;
+  }
+
+  const T *_Nullable operator ->(void) const & {
+    return get();
+  }
+};
+
 /// A type representing a pointer relative to itself with low bits reserved for
 /// use as flags.
 ///
@@ -98,6 +118,13 @@ struct SWTRelativePointerIntPair: public SWTRelativePointer<T, maskValue> {
   }
 };
 
+template <typename T>
+#if defined(__wasm32__)
+using SWTCompactFunctionPointer = SWTAbsoluteFunctionPointer<T>;
+#else
+using SWTCompactFunctionPointer = SWTRelativePointer<T>;
+#endif
+
 /// A type representing a metatype as constructed during compilation of a Swift
 /// module.
 ///
@@ -114,7 +141,7 @@ struct SWTTypeContextDescriptor {
     size_t state;
   };
   using MetadataAccessFunction = __attribute__((swiftcall)) MetadataAccessResponse(size_t);
-  SWTRelativePointer<MetadataAccessFunction> _metadataAccessFunction;
+  SWTCompactFunctionPointer<MetadataAccessFunction> _metadataAccessFunction;
 
 public:
   const char *_Nullable getName(void) const& {