There are a few places that will remain std::unordered_map due to build
or run-time errors, which will need to be evaluated more closely to
determine if they should remain std::unordered_map or if there is
another Abseil data structure that would be more efficient while still
working correctly.

Bug: angleproject:4873
Change-Id: Ib04253e3ad6398e63f4cc2bfe12c0f9e57cb112b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2558873
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>