Serialize `pragma ms_struct` state.

Serialize `pragma ms_struct` state.

pragma ms_struct has an effect on struct decls, and the effect is serialized
correctly already.  But the "is ms_struct currently on" state wasn't before
this change.

This uses the same approach as `pragma clang optimize`: When writing a module,
the state isn't serialized, only when writing a pch file.

llvm-svn: 262539