Revert r193223 and r193216.

They were causing CodeGenCXX/mangle-exprs.cpp to fail.

Revert "Remove the circular reference to LambdaExpr in CXXRecordDecl."

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193226 91177308-0d34-0410-b5e6-96231b3b80d8