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>