Pull the mutable semantic fields from ast::Function and into a new semantic::Function node.
Have the TypeDeterminer create these semantic::Function nodes.
Bug: tint:390
Change-Id: I237b1bed8709dd9a3cfa24d85d48fc77b7e532da
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/39902
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>