HLSL: Fix handling nested structs in interface blocks Make sure that the type definitions for nested structs get added to the HLSL header, and that the std140 padding information gets recorded. Prior to this trying to use nested structs in interface blocks crashed. BUG=angleproject:2084 TEST=angle_end2end_tests Change-Id: If57870285c6feaf0c2e462f98f50f20730dd6470 Reviewed-on: https://chromium-review.googlesource.com/608449 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>