Add missing benchmark test.

There is a benchmark test for builtin_value which was not added
into the benchmark code. This cl adds the missing test.

Change-Id: I6fc0eee0a50cad31f516024125be11e434df0ac0
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/108461
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>

src/tint/CMakeLists.txt

@@ -1364,6 +1364,7 @@ if(TINT_BUILD_BENCHMARKS)
 
   set(TINT_BENCHMARK_SRC
     "castable_bench.cc"
+    "ast/builtin_value_bench.cc"
     "ast/extension_bench.cc"
     "ast/address_space_bench.cc"
     "ast/texel_format_bench.cc"