Useful to construct a FastVector out of a subrange of another container.

Bug: angleproject:4889
Change-Id: I0e62b601c7d171167343d526d198fa21ba52f0f9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2920191Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>