Prerequisite code for simpler GlslangWrapper. This makes two changes for the linking step: * allows symbol names in TOutputGLSLBase::writeVariableType * stores a list of inactive varying names in VaryingPacking Bug: angleproject:2456 Change-Id: Id651721a008c049508303d18c45c6b96c76114d9 Reviewed-on: https://chromium-review.googlesource.com/1014707 Reviewed-by: Luc Ferron <lucferron@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>