Vulkan: SPIR-V Gen: Support function forward declaration The generation of function-related ids is moved to visitFunctionPrototype to support forward declarations. The DeferGlobalInitializers AST transformation produces such code. Bug: angleproject:4889 Change-Id: Iefee534575092ca25ff86e416e35703d7022bc07 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2939332 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>