[stdlib] Annotate types with @_fixed_layout

This will allows us to build the standard library in resilient mode by
default, hopefully, without performance regression.

<rdar://problem/36362648>

stdlib/public/core/UnicodeParser.swift

@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 extension Unicode {
   /// The result of attempting to parse a `T` from some input.
+  @_fixed_layout // FIXME(sil-serialize-all)
   public enum ParseResult<T> {
   /// A `T` was parsed successfully
   case valid(T)