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-inner-block.wgsl

@@ -12,9 +12,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+[[stage(fragment)]]
 fn main() -> void {
     var a : f32 = 2.0;
    { a = 3.14;}
     return;
 }
-entry_point fragment = main;