To support Program Pipeline Objects, the SPIR-V transformations must be
completed at draw time once all of the variable locations are known. To
achieve this, the transformation work is being refactored into
TransformSpirvCode() which can be called separately from
GlslangGetShaderSpirvCode().

Bug: angleproject:3570
Change-Id: Ia479cb2e4b4201e37acd0859c70081cecb80d0bf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2068900
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>