[clang] Don't parse MS attributes in `ParseExportDeclaration()`.

As @rsmith commented on https://reviews.llvm.org/D111548: "That looks like it's
simply a bug as far as I can tell, and that call can be removed. MS attributes
will be parsed as part of the decl specifier sequence as needed and don't need
to be parsed as declaration attributes."

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D126062