RewriteBuffer: Remove an unused typedef

This hasn't been used for some time.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194191 91177308-0d34-0410-b5e6-96231b3b80d8
Tests for DR150 - DR165.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194192 91177308-0d34-0410-b5e6-96231b3b80d8
Mark generic lambdas, and thus C++14, as complete. Add commented-out stub for C++17.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194194 91177308-0d34-0410-b5e6-96231b3b80d8