FastIntegerSet::clear() now calls reset() instead of assign().
std::vector::assign(...) invokes the copy-constructor. For better
performance, especially with BitSet*, we can leverage the reset()
method.

This removes 1.2% CPU overhead from a Manhattan30 offscreen run

Bug: angleproject:5689
Change-Id: Ib1a760587ffe18341b8ed892a732c506fc50c82b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2723494
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>