Revert "Pin to LLVM's legacy pass manager"

This reverts commit 2ab55b52e5041aa459ffd2a0584eb035b646149d.

Reason for revert:

Findit (https://goo.gl/kROfz5) identified CL at revision 849311 as the
culprit for failures in the build cycles as shown on:
https://analysis.chromium.org/waterfall/culprit?key=ag9zfmZpbmRpdC1mb3ItbWVyRAsSDVdmU3VzcGVjdGVkQ0wiMWNocm9taXVtLzJhYjU1YjUyZTUwNDFhYTQ1OWZmZDJhMDU4NGViMDM1YjY0NjE0OWQM

Sample Failed Build: https://ci.chromium.org/b/8856456307405297840

Sample Failed Step: compile

Original change's description:
> Pin to LLVM's legacy pass manager
> 
> So we can switch pass managers independently of a clang roll.
> 
> Bug: 1173188
> Change-Id: I788a3342fa795fa39495acb4477c0af963ad21b5
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2660563
> Commit-Queue: Arthur Eubanks <aeubanks@google.com>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#849311}


Change-Id: Id63c0cc2acb9a85824691a8312951bd93e1cc3d5
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1173188
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2666049
Cr-Commit-Position: refs/heads/master@{#849329}