GLSL: implement builtin kNumWorkgroups.

Bug: tint:1400
Change-Id: I5f2523d41f73abe55e57b8931833124c2ebaf00d
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/78260
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>

src/writer/glsl/generator_impl.cc

@@ -1787,6 +1787,7 @@ sem::Type* GeneratorImpl::builtin_type(ast::Builtin builtin) {
     }
     case ast::Builtin::kLocalInvocationId:
     case ast::Builtin::kGlobalInvocationId:
+    case ast::Builtin::kNumWorkgroups:
     case ast::Builtin::kWorkgroupId: {
       auto* u32 = builder_.create<sem::U32>();
       return builder_.create<sem::Vector>(u32, 3);
@@ -1828,6 +1829,8 @@ const char* GeneratorImpl::builtin_to_string(ast::Builtin builtin,
       return "gl_LocalInvocationIndex";
     case ast::Builtin::kGlobalInvocationId:
       return "gl_GlobalInvocationID";
+    case ast::Builtin::kNumWorkgroups:
+      return "gl_NumWorkGroups";
     case ast::Builtin::kWorkgroupId:
       return "gl_WorkGroupID";
     case ast::Builtin::kSampleIndex: