Implement CapturedStmt AST

CapturedStmt can be used to implement generic function outlining as described in
http://lists.cs.uiuc.edu/pipermail/cfe-dev/2013-January/027540.html.

CapturedStmt is not exposed to the C api.

Serialization and template support are pending.

Author: Wei Pan <wei.pan@intel.com>

Differential Revision: http://llvm-reviews.chandlerc.com/D370


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179615 91177308-0d34-0410-b5e6-96231b3b80d8