tint ast fuzzer: look for dxc in default path if not passed in via op…

…tions

If dxc path is not passed in, look for it first by name. This was the
original behaviour, but was accidentally modified in:
https://dawn-review.googlesource.com/c/dawn/+/190920

Bug: 380270956
Change-Id: I959d73868cb6d0a8997f7cc0624b96bf515c1c37
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/218317
Reviewed-by: James Price <[email protected]>
Commit-Queue: Antonio Maiorano <[email protected]>

src/tint/lang/hlsl/writer/writer_ast_fuzz.cc

@@ -80,9 +80,14 @@ void ASTFuzzer(const tint::Program& program,
     constexpr bool must_validate = false;
 
     const char* dxc_path = validate::kDxcDLLName;
-    Result<tint::hlsl::writer::Output> res;
     if (!context.options.dxc.empty()) {
         dxc_path = context.options.dxc.c_str();
+    }
+    auto dxc = tint::Command::LookPath(dxc_path);
+
+    Result<tint::hlsl::writer::Output> res;
+    if (dxc.Found()) {
+        // If validating with DXC, run renamer transform first to avoid DXC validation failures.
         ast::transform::DataMap inputs, outputs;
         inputs.Add<ast::transform::Renamer::Config>(ast::transform::Renamer::Target::kHlslKeywords);
         if (auto renamer_res = tint::ast::transform::Renamer{}.Apply(program, inputs, outputs)) {
@@ -91,7 +96,6 @@ void ASTFuzzer(const tint::Program& program,
             }
             res = tint::hlsl::writer::Generate(*renamer_res, options);
         }
-
     } else {
         res = tint::hlsl::writer::Generate(program, options);
     }
@@ -104,7 +108,6 @@ void ASTFuzzer(const tint::Program& program,
         std::cout << "Dumping generated HLSL:\n" << res->hlsl << std::endl;
     }
 
-    auto dxc = tint::Command::LookPath(dxc_path);
     if (dxc.Found()) {
         uint32_t hlsl_shader_model = 60;
         bool require_16bit_types = false;