Since we might want to use the ANGLE PoolAlloc in the Vulkan back-end,
we want to make completely sure that it doesn't show up in any header
that might be included with the Glslang headers, since this could
cause a conflict.

This change moves as much as possible to forward-declaring instead of
including headers directly in ProgramLinkedResources.h. This means
making several internal Program helper classes external. I.E. instead
of Program::Bindings, we have ProgramBindings.

It also redeclares a "using" in two places, since it isn't possible to
forward declare these, and it seemed overdesigned to put the using in
a separate header.

Bug: angleproject:2264
Change-Id: Idd08706580c927327dddf46e86acbcd2c4e3286f
Reviewed-on: https://chromium-review.googlesource.com/792270Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>