[WebGPU] Support writing @Builtin(frag_depth) from the fragment shader

https://bugs.webkit.org/show_bug.cgi?id=252452
rdar://105578867

Reviewed by Mike Wyrzykowski.

During code generation we were skipping serialization of builtins for fragment
output struct members.

* Source/WebGPU/WGSL/Metal/MetalFunctionWriter.cpp:
(WGSL::Metal::FunctionDefinitionWriter::visit):
* Source/WebGPU/WGSL/tests/valid/fragment-output.wgsl:

Canonical link: https://commits.webkit.org/269929@main

Source/WebGPU/WGSL/Metal/MetalFunctionWriter.cpp

@@ -505,10 +505,9 @@ void FunctionDefinitionWriter::visit(AST::BuiltinAttribute& builtin)
     // Built-in attributes are only valid for parameters. If a struct member originally
     // had a built-in attribute it must have already been hoisted into a parameter, but
     // we keep the original struct so we can reconstruct it.
-    if (m_structRole.has_value() && *m_structRole != AST::StructureRole::VertexOutput)
+    if (m_structRole.has_value() && *m_structRole != AST::StructureRole::VertexOutput && *m_structRole != AST::StructureRole::FragmentOutput)
         return;
 
-    // FIXME: we should replace this with something more efficient, like a trie
     switch (builtin.builtin()) {
     case Builtin::FragDepth:
         m_stringBuilder.append("[[depth(any)]]");

Source/WebGPU/WGSL/tests/valid/fragment-output.wgsl

@@ -1,5 +1,6 @@
 // RUN: %metal-compile main
 struct S {
+  @builtin(frag_depth) depth: f32,
   @location(0) x : vec3<f32>,
   @location(1) y : vec4<f32>,
 }
@@ -11,5 +12,6 @@ fn main() -> S {
   var output : S;
   output.x = vec3f(0);
   output.y = vec4f(1);
+  output.depth = 1;
   return output;
 }