Remove pessimized usages of MakeFlatMap and MakeFlatSet

These call sites try to move a container into MakeFlatMap or MakeFlatSet
without transforming it (due to the use of the identity projection and
lack of implicit conversions), but those utilities take the input
container by const reference, which means that the container isn't moved
at all, resulting in both pointless copies of the individual elements
and allocation of a separate container.

We replace these unnecessary calls with direct uses of the flat_map
and flat_set constructors, which do accept containers by move.

Ideally MakeFlatMap and MakeFlatSet would be adjusted to prevent this
pessimization automatically, either by making this situation a
compile-time error or by accepting a forwarding reference with some
kind of dispatch based on whether the projection is the identity, but
that is outside the scope of this CL.

Change-Id: Id94c6cb18082b7fbd10e1a4ed072199c8c16851f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5982875
Commit-Queue: Andrew Paseltiner <apaseltiner@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Russ Hamilton <behamilton@google.com>
Cr-Commit-Position: refs/heads/main@{#1377668}