Previously we had only supported simple integer constants, as that was
enough to parse array declarations etc. Fully implement constants,
including large null objects, and constant composites constructed out
of other constants.

Bug: b/124934655
Change-Id: Ia1087f62a2bd75a3ca2f784ea24b0b3f01984ad9
Reviewed-on: https://swiftshader-review.googlesource.com/c/25148Tested-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>