Includes only the include/spirv/unified1 subtree.

Based on git://github.com/KhronosGroup/spirv-tools revision 17da9f8231f78cf519b4958c2229463a63ead9e2

Bug: b/120799499
Change-Id: Ifb790bf95035d7feb2d54d07e1475013dfd298b1
Reviewed-on: https://swiftshader-review.googlesource.com/c/23048Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Corentin Wallez <cwallez@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>