Small CapturedStmt improvements

Add a CapturedStmt.h similar to Lambda.h to reduce the typing required to get
to the CapturedRegionKind enum. This also allows codegen to access this enum
without including Sema/ScopeInfo.h.

Also removes some duplicated code for capturing 'this' between CapturedStmt and
Lambda.

Differential Revision: http://llvm-reviews.chandlerc.com/D712