Serialization for captured statements

Add serialization for captured statements and captured decls.  Also add
a const_capture_iterator to CapturedStmt.

Test contributed by Wei Pan

Differential Revision: http://llvm-reviews.chandlerc.com/D727


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181048 91177308-0d34-0410-b5e6-96231b3b80d8