[gn] port b712aef

Author: nico

llvm/utils/gn/secondary/clang/test/BUILD.gn

@@ -65,6 +65,7 @@ write_lit_config("lit_site_cfg") {
     "LLVM_EXTERNAL_LIT=",
     "LLVM_HOST_TRIPLE=$llvm_current_triple",
     "LLVM_LIT_TOOLS_DIR=",  # Intentionally empty, matches cmake build.
+    "LLVM_TOOL_LLVM_DRIVER_BUILD=0",  # FIXME: Add actual support for this.
     "LLVM_USE_SANITIZER=",
     "Python3_EXECUTABLE=$python_path",
     "USE_Z3_SOLVER=",