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.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@262539 91177308-0d34-0410-b5e6-96231b3b80d8