Teach TreeTransform and family how to transform generic lambdas within templates and nested within themselves.
  
This does not yet include capturing (that is next).

Please see test file for examples.

This patch was LGTM'd by Doug:
http://llvm-reviews.chandlerc.com/D1784


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191875 91177308-0d34-0410-b5e6-96231b3b80d8