Introduce support for template instantiation of lambda

expressions. This is mostly a simple refact, splitting the main "start
a lambda expression" function into smaller chunks that are driven
either from the parser (Sema::ActOnLambdaExpr) or during AST
transformation (TreeTransform::TransformLambdaExpr). A few minor
interesting points:

  - Added new entry points for TreeTransform, so that we can
  explicitly establish the link between the lambda closure type in the
  template and the lambda closure type in the instantiation.
  - Added a bit into LambdaExpr specifying whether it had an explicit
  result type or not. We should have had this anyway.

This code is 'lightly' tested.

llvm-svn: 150417