Update no include guard warning suppression for instant_struct_traits.h

Switch from an explanatory comment to the string
'no-include-guard-because-multiply-included' to stop
the warning every time the file is edited.

Also add NOLINT(build/header_guard) to stop Tricium from flagging the
same problem.


Change-Id: Iccaf808919606c36ff2c770b44e39b6ba6504bae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1582459
Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657349}