[gn build] (manually) port 5390606a

llvm/utils/gn/secondary/clang/include/clang/Config/BUILD.gn

@@ -25,6 +25,7 @@ write_cmake_config("Config") {
     "CLANG_CONFIG_FILE_SYSTEM_DIR=",
     "CLANG_CONFIG_FILE_USER_DIR=",
     "CLANG_SPAWN_CC1=",
+    "CLANG_ENABLE_OPAQUE_POINTERS_INTERNAL=",
     "DEFAULT_SYSROOT=",
     "GCC_INSTALL_PREFIX=",
     "BACKEND_PACKAGE_STRING=LLVM ${llvm_version}git",