Move the Extension builtin to the builtin/ folder.

This CL moves Extension from ast:: to builtin::

Change-Id: Ica1e6f4f9229341b547011389d47ae3be9830b0d
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/120362
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
diff --git a/src/tint/resolver/variable_validation_test.cc b/src/tint/resolver/variable_validation_test.cc
index 2649562..738d926 100644
--- a/src/tint/resolver/variable_validation_test.cc
+++ b/src/tint/resolver/variable_validation_test.cc
@@ -488,7 +488,7 @@
 TEST_F(ResolverVariableValidationTest, GlobalVariable_PushConstantWithInitializer) {
     // enable chromium_experimental_push_constant;
     // var<push_constant> a : u32 = 0u;
-    Enable(ast::Extension::kChromiumExperimentalPushConstant);
+    Enable(builtin::Extension::kChromiumExperimentalPushConstant);
     GlobalVar(Source{{1u, 2u}}, "a", ty.u32(), type::AddressSpace::kPushConstant,
               Expr(Source{{3u, 4u}}, u32(0)));