Isolate GlslangWrapper header from most of ANGLE. 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/792270 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>