HLSL output maps structs in std140 uniform blocks to a different
layout in order to eliminate padding. The padding may have been
inserted to comply with std140 packing rules.

There used to be two issues in writing the maps: Sometimes the same
map could be written multiple times, and the maps were not being
written for uniform blocks with instance names.

Rewrite how the uniform buffer struct maps get generated so that
the code works correctly. Instead of flagging accesses, structs inside
uniform blocks are gathered from uniform block declarations. When
accesses to structs in uniform blocks are written out in OutputHLSL,
it's checked whether a mapped struct needs to be used instead of the
original one.

This code could still be optimized further by limiting mapped structs
generation to those ones that really need to be used. This is left to
be done later.

BUG=angleproject:2084
TEST=angle_end2end_tests

Change-Id: Iee24b3ef15847d2af64554ac74b8e4be5060d18c
Reviewed-on: https://chromium-review.googlesource.com/751506Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>