Split descriptor decorations from other decorations since they're always
used separately and this makes propagating them more lightweight.

Bug b/129523279

Change-Id: I2f5acecc990bf15c7feb4ce81539bd2b5818bdf6
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/28445
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolas.capens@gmail.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>