Remove support for the old _idx builtins.

This CL removes the old _idx variants of builtins in favour of the
_index versions.

Change-Id: I1678b5fae63c9d4f7f63feabf7410a58505d397a
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/38825
Auto-Submit: dan sinclair <dsinclair@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: dan sinclair <dsinclair@chromium.org>

src/reader/wgsl/parser_impl.cc

diff --git a/src/reader/wgsl/parser_impl.cc b/src/reader/wgsl/parser_impl.cc
index 649589b..dcf8794 100644
--- a/src/reader/wgsl/parser_impl.cc
+++ b/src/reader/wgsl/parser_impl.cc
@@ -103,10 +103,10 @@
   if (str == "position") {
     return ast::Builtin::kPosition;
   }
-  if (str == "vertex_idx" || str == "vertex_index") {
+  if (str == "vertex_index") {
     return ast::Builtin::kVertexIndex;
   }
-  if (str == "instance_idx" || str == "instance_index") {
+  if (str == "instance_index") {
     return ast::Builtin::kInstanceIndex;
   }
   if (str == "front_facing") {
@@ -121,7 +121,7 @@
   if (str == "local_invocation_id") {
     return ast::Builtin::kLocalInvocationId;
   }
-  if (str == "local_invocation_idx" || str == "local_invocation_index") {
+  if (str == "local_invocation_index") {
     return ast::Builtin::kLocalInvocationIndex;
   }
   if (str == "global_invocation_id") {

src/reader/wgsl/parser_impl_variable_decoration_test.cc

diff --git a/src/reader/wgsl/parser_impl_variable_decoration_test.cc b/src/reader/wgsl/parser_impl_variable_decoration_test.cc
index 128ec8a..db50003 100644
--- a/src/reader/wgsl/parser_impl_variable_decoration_test.cc
+++ b/src/reader/wgsl/parser_impl_variable_decoration_test.cc
@@ -113,17 +113,13 @@
     ParserImplTest,
     BuiltinTest,
     testing::Values(BuiltinData{"position", ast::Builtin::kPosition},
-                    BuiltinData{"vertex_idx", ast::Builtin::kVertexIndex},
                     BuiltinData{"vertex_index", ast::Builtin::kVertexIndex},
-                    BuiltinData{"instance_idx", ast::Builtin::kInstanceIndex},
                     BuiltinData{"instance_index", ast::Builtin::kInstanceIndex},
                     BuiltinData{"front_facing", ast::Builtin::kFrontFacing},
                     BuiltinData{"frag_coord", ast::Builtin::kFragCoord},
                     BuiltinData{"frag_depth", ast::Builtin::kFragDepth},
                     BuiltinData{"local_invocation_id",
                                 ast::Builtin::kLocalInvocationId},
-                    BuiltinData{"local_invocation_idx",
-                                ast::Builtin::kLocalInvocationIndex},
                     BuiltinData{"local_invocation_index",
                                 ast::Builtin::kLocalInvocationIndex},
                     BuiltinData{"global_invocation_id",