Revert "[llvm] Add initial scaffolding"

This reverts commit b71d2d99965e7f283cb153f96a9f9b2bb2d3197d.

Reason for revert: Somehow this broke flutter because gclient doesn't seem to have generated the needed file

Original change's description:
> [llvm] Add initial scaffolding
> 
> This change adds the gclient and GN changes needed to build
> an executable using LLVM in the Dart tree as well as a basic
> testing framework based on llvm-lit.
> 
> Change-Id: I9009a98ff95043cc3754966f31697ba7f1712310
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/106434
> Commit-Queue: Jake Ehrlich <jakehehrlich@google.com>
> Reviewed-by: Alexander Thomas <athom@google.com>
> Reviewed-by: Vyacheslav Egorov <vegorov@google.com>

TBR=vegorov@google.com,kustermann@google.com,athom@google.com,phosek@google.com,ajcbik@google.com,jakehehrlich@google.com

Change-Id: I877abfe211dc6c3efd94cc2350d028bf54df9d9f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/107688
Reviewed-by: Jake Ehrlich <jakehehrlich@google.com>
Commit-Queue: Jake Ehrlich <jakehehrlich@google.com>