message: Fix build with older (<3.0) swigs
  It turns out it wasn't the compilers, but swig who had issues with my
  previous patch -- older versions do not recognise the "constexpr"
  keyword.
  Fortunately, that can be fixed the same way we fix all other swig
  incompatibilities: #ifndef SWIG.
diff: #include <type_traits>
  +#ifndef SWIG
  // Macro to enable bitmask operations on an enum. Without this, Enum | Enum
  // gets promoted to an int, so you have to say Enum a = Enum(eFoo | eBar). If
  // you mark Enum with LLDB_MARK_AS_BITMASK_ENUM(Enum), however, you can simply
  // write Enum a = eFoo | eBar.
  +// Unfortunately, swig<3.0 doesn't recognise the constexpr keyword, so remove
  +// this entire block, as it is not necessary for swig processing.
  #define LLDB_MARK_AS_BITMASK_ENUM(Enum) \
  constexpr Enum operator|(Enum a, Enum b) { \
  return static_cast<Enum>( \
  a = a & b; \
  return a; \
  }
  +#else
  +#define LLDB_MARK_AS_BITMASK_ENUM(Enum)
  +#endif
  #ifndef SWIG
  // With MSVC, the default type of an enum is always signed, even if one of the
  