Parser support for #pragma clang __debug captured

This patch implements parsing ‘#pragma clang __debug’ as a first step for
implementing captured statements. Captured statements are a mechanism for
doing outlining in the AST.
see http://lists.cs.uiuc.edu/pipermail/cfe-dev/2013-January/027540.html.

Currently returns StmtEmpty

Author: Andy Zhang <andy.zhang@intel.com>

Differential Revision: http://llvm-reviews.chandlerc.com/D369