Revert "Teach TreeTransform and family how to transform generic lambdas within templates and nested within themselves."

This reverts commit r191879. It caused llvm.org/pr17476.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191955 91177308-0d34-0410-b5e6-96231b3b80d8