Move the similar function merging pass into the LLVM repo

[vedantk]

This makes it easier to experiment with function merging in clang.

This set of PRs just does pure code movement.

Depends on:
apple/swift-llvm#123
apple/swift-clang#231

rdar://45707083

[vedantk]

apple/swift-llvm#123
apple/swift-clang#231
@swift-ci please smoke test

[aschwaighofer]

LGTM

[eeckstein]

lgtm!

[vedantk]

apple/swift-llvm#123
apple/swift-clang#231
@swift-ci please smoke test

[vedantk]

apple/swift-llvm#123
apple/swift-clang#231
@swift-ci please smoke test

[vedantk]

@eeckstein @aschwaighofer thanks for the reviews!

Here's my plan for landing this:

1. Commit the llvm & clang parts to github/upstream-with-swift, then cherry-pick to swift-5.0-branch.
2. Wait until the changes are auto-merged into the swift-{llvm,clang}/stable branches. This can't be forced -- those branches are gated.
3. Merge this PR to delete swift's copy of the pass.

Does that sound right?

[vedantk]

On second thought, it might be more difficult to maintain a copy of this pass in the upstream-with-swift branch, as it would require more effort to keep it in sync with llvm.org. Let's wait until we can really upstream this pass to do this.