don't merge entire env when running dxil --version

Author: bob80905

tools/clang/test/lit.cfg

@@ -517,10 +517,10 @@ if config.metal:
 
 # Check supported dxil version
 def get_dxil_version():
-    # Merge current environment with lit's config.environment
-    env = os.environ.copy()
-    env.update(config.environment)
-    result = subprocess.run([lit.util.which('dxc', llvm_tools_dir), "--version"], stdout=subprocess.PIPE, env=env)
+    if "DXC_DXIL_DLL_PATH" in config.environment:
+        result = subprocess.run([lit.util.which('dxc', llvm_tools_dir), "--version"], stdout=subprocess.PIPE, env=config.environment)
+    else:
+        result = subprocess.run([lit.util.which('dxc', llvm_tools_dir), "--version"], stdout=subprocess.PIPE)
     output = result.stdout.decode("utf-8")
     dxcPat = re.compile(r"(dxcompiler.dll|libdxcompiler.so|libdxcompiler.dylib): (?P<dxcMajor>[0-9]+)\.(?P<dxcMinor>[0-9]+).")
     m = dxcPat.search(output)