Remove EntryPoint.

This CL removes the EntryPoint node and transitions everything to the
stage decoration.

Change-Id: Ib2840155905c8fa60ff35870f0c4b6705efb73ff
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/28705
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Reviewed-by: David Neto <dneto@google.com>
Reviewed-by: Sarah Mashayekhi <sarahmashay@google.com>

test/undefined-variable-global-scope.wgsl

@@ -15,9 +15,9 @@
 # Vertex shader
 var<private> a : i32;
 
+[[stage(vertex)]]
 fn main() -> void {
   a = 2;
   return;
 }
-entry_point vertex as "main" = main;