message: Tweak comment on XlaClusterInfo's default constructor.
diff: @@ -117,11 +117,14 @@ Status PreprocessForEncapsulation(Graph* g,
  // Information for XLA computation.
  struct XlaClusterInfo {
  - // The implicit default constructor is deleted because host_compute_core is a
  - // const member whose type (std::map) doesn't necessarily have a user provided
  - // constructor - while libc++ and libstdc++ 4.8 provide a user defined
  - // default constructor, libstdc++ at least >= 7.3 does not.
  - // See also c++11 [class.ctor] p5.
  + // Add an explicitly-defined default constructor for this class.
  + //
  + // The compiler may delete the default constructor here because
  + // host_compute_core is a const member whose type (std::map) doesn't
  + // necessarily have a user provided constructor -- while libc++ and
  + // libstdc++ 4.8 provide a user defined default constructor, libstdc++ at
  + // least >= 7.3 does not. See also c++11 [class.ctor] p5.
  + //
  // TODO(klimek): In c++17 we'll be able to initialize host_compute_core
  // without losing aggregate initialization, which allows us to get rid of
  // the constructor definitions again.
  