[ASTDumper][OpenMP] CapturedDecl has a 'nothrow' bit

Summary:
Was trying to understand how complicated it would be to write
a clang-tidy `openmp-exception-escape`-ish check once D57100 lands.

Just so it happens, all the data is already there,
it is just conveniently omitted from AST dump.

Reviewers: aaron.ballman, steveire, ABataev

Reviewed By: ABataev

Subscribers: ABataev, guansong, cfe-commits

Tags: #openmp, #clang

Differential Revision: https://reviews.llvm.org/D57452

llvm-svn: 352631