This adds diagnostic warnings for packed cache structures. It ensures
the packed versions of the structures don't have any unexpected
misalignments or inserted members. This gives us consistent
behaviour and ensures all memory is initialized.

Implemented for Clang/GCC/MSVC.

Bug: angleproject:2522
Change-Id: I6ec453a40d292e4a498319ffa767988a502d225e
Reviewed-on: https://chromium-review.googlesource.com/c/1302533Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>