This new node type can be used to inject preprocessor directives into
the intermediate tree. Outputting the preprocessor directive is easy
for the GLSL back-end. This can be used to implement workarounds such
as the OpenGL line rasterization rules on Vulkan.

Also fixes the build that was broken on Win/x64 with a prior change and
makes more methods in IntermNode.h const.

Bug: angleproject:2598
Change-Id: Ifd6d0ac7912ccf2137997bb9a0187fd063023d5e
Reviewed-on: https://chromium-review.googlesource.com/1133420Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>