This relands commit a79f9031.

Original change's description:
> [Interpreter] Unify approach to building interpreter handler and Turbofan stubs.
> 
> Moves interpreter-generator.cc to a similar model of building handlers as
> Turbofan stubs elsewhere, to simplify moving code between stubs / builtins and
> bytecode handlers. This removes the "__" hack from the Interpreter generator
> code.
> 
> Also make SetBytecodeOffset private to InterpreterAssembler and make 
> LdaImmutable[Current]ContextSlot and Lda[Current]ContextSlot share
> handlers since they are identical.
> 
> Change-Id: I9e91e7d37c2ea75513e4dcc3b95b4bb6517f83da
> Reviewed-on: https://chromium-review.googlesource.com/471987
> Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
> Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
> Reviewed-by: Igor Sheludko <ishell@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#44534}
> 
TBR=rmcilroy@chromium.org,jkummerow@chromium.org,machenbach@chromium.org,cbruni@chromium.org,leszeks@chromium.org,v8-reviews@googlegroups.com,ishell@chromium.org

Change-Id: I282fe5582f681ccb0642537a70f89185558ee195
Reviewed-on: https://chromium-review.googlesource.com/474755Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44617}