Sema for Captured Statements

Add CapturedDecl to be the DeclContext for CapturedStmt, and perform semantic
analysis. Currently captures all variables by reference.

TODO: templates

Author: Ben Langmuir <ben.langmuir@intel.com>

Differential Revision: http://llvm-reviews.chandlerc.com/D433


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179618 91177308-0d34-0410-b5e6-96231b3b80d8