Fix an ABI break

PR swiftlang#74394 removed a field from `struct MetadataSections` whose layout is ABI.
So restore that field as an unused padding field to preserve the layout of all
the other fields in that struct.

Author: tbkka

stdlib/public/SwiftShims/swift/shims/MetadataSections.h

@@ -95,6 +95,7 @@ struct MetadataSections {
   MetadataSectionRange swift5_replac2;
   MetadataSectionRange swift5_builtin;
   MetadataSectionRange swift5_capture;
+  MetadataSectionRange swift5_mpenum_unused; // No longer used
   MetadataSectionRange swift5_accessible_functions;
   MetadataSectionRange swift5_runtime_attributes;
   MetadataSectionRange swift5_tests;