[spirv-writer] Emit entrypoint from function decoration.

This CL updates the SPIRV-Writer to emit entry point information based
on the function stage as well as EntryPoint nodes.

Change-Id: I1fa937cbb2159b31516b0189216d679e03f0384d
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/28702
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: dan sinclair <dsinclair@chromium.org>

test/function.wgsl

@@ -21,4 +21,3 @@ fn main() -> f32 {
 fn ep() -> void {
   return;
 }
-entry_point compute = ep;