[NFC] Minor conveniences for working with metadata.

Author: rjmccall

include/swift/ABI/Metadata.h

@@ -625,11 +625,27 @@ struct TargetMetadata {
     getValueWitnesses()->_asEVWT()->destructiveInjectEnumTag(value, tag, this);
   }
 
+  size_t vw_size() const {
+    return getValueWitnesses()->getSize();
+  }
+
+  size_t vw_alignment() const {
+    return getValueWitnesses()->getAlignment();
+  }
+
+  size_t vw_stride() const {
+    return getValueWitnesses()->getStride();
+  }
+
   /// Allocate an out-of-line buffer if values of this type don't fit in the
   /// ValueBuffer.
   /// NOTE: This is not a box for copy-on-write existentials.
   OpaqueValue *allocateBufferIn(ValueBuffer *buffer) const;
 
+  /// Get the address of the memory previously allocated in the ValueBuffer.
+  /// NOTE: This is not a box for copy-on-write existentials.
+  OpaqueValue *projectBufferFrom(ValueBuffer *buffer) const;
+
   /// Deallocate an out-of-line buffer stored in 'buffer' if values of this type
   /// are not stored inline in the ValueBuffer.
   void deallocateBufferIn(ValueBuffer *buffer) const;

stdlib/public/runtime/Metadata.cpp

@@ -3712,6 +3712,13 @@ template <> OpaqueValue *Metadata::allocateBufferIn(ValueBuffer *buffer) const {
   return reinterpret_cast<OpaqueValue *>(ptr);
 }
 
+template <> OpaqueValue *Metadata::projectBufferFrom(ValueBuffer *buffer) const{
+  auto *vwt = getValueWitnesses();
+  if (vwt->isValueInline())
+    return reinterpret_cast<OpaqueValue *>(buffer);
+  return reinterpret_cast<OpaqueValue *>(buffer->PrivateData[0]);
+}
+
 template <> void Metadata::deallocateBufferIn(ValueBuffer *buffer) const {
   auto *vwt = getValueWitnesses();
   if (vwt->isValueInline())