[DeclPrinter] Fix two cases that crash clang -ast-print.

Both are related to handling anonymous structures.
* clang didn't handle () around an anonymous struct variable.
* clang also crashed on syntax errors that could lead to other
  syntactic constructs following the declaration of an
  anonymous struct. While the code is invalid, that's not
  a good reason to panic compiler.

Differential Revision: https://reviews.llvm.org/D41788

llvm-svn: 322742