This CL contains a few pieces:

- A new mechanism to create "BuiltinContinuation" checkpoints in TurboFan
  graphs, which--when triggered--swizzle the values in the the FrameState to be
  parameters to a typically TF-generated builtin that resumes execution to finish
  the slow-case functionality.
- Continuation builtins that have special handling in the deoptimizer and their own
  new frame type to ensure that the values they need to begin executing can be stashed
  away and restored immediately before the builtin is called via a trampoline that runs
  when the continuation builtin's frame execution resumes.
- An implementation of Array.prototype.forEach in TurboFan that can be used to
  inline it. The inlined forEach implementation uses the checkpoints mechanism
  described above to deopt in the middle of the forEach in the cases that optimization
  invariants are violated. There is a slightly different continuation stub for each
  deopt point in the forEach implementation to ensure the correct side-effects, i.e.
  that the deopt of the builtin isn't programmatically observable.

Review-Url: https://codereview.chromium.org/2803853005
Cr-Commit-Position: refs/heads/master@{#45764}