[gn build] Disable objc rewriter (#155479)

This is off by default in the CMake build:
https://github.com/llvm/llvm-project/blob/b90f4ff3020c6fe018ec22b22a7faa5541ed5372/clang/CMakeLists.txt#L441

Author: aeubanks

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

@@ -25,7 +25,7 @@ write_cmake_config("Config") {
     "GCC_INSTALL_PREFIX=",
     "ENABLE_LINKER_BUILD_ID=",
     "ENABLE_X86_RELAX_RELOCATIONS=1",
-    "CLANG_ENABLE_OBJC_REWRITER=1",  # FIXME: flag?
+    "CLANG_ENABLE_OBJC_REWRITER=",
     "CLANG_ENABLE_CIR=",
     "CLANG_SYSTEMZ_DEFAULT_ARCH=z10",
     "PPC_LINUX_DEFAULT_IEEELONGDOUBLE=",

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

@@ -54,7 +54,7 @@ write_lit_config("lit_site_cfg") {
     "CLANG_DEFAULT_CXX_STDLIB=",  # Empty string means "default value" here.
     "CLANG_DEFAULT_PIE_ON_LINUX=0",
     "CLANG_ENABLE_CIR=0",
-    "CLANG_ENABLE_OBJC_REWRITER=1",  # FIXME: flag?
+    "CLANG_ENABLE_OBJC_REWRITER=0",
     "CLANG_PLUGIN_SUPPORT=0",  # FIXME: Analysis/plugins need global -fPIC
     "CURRENT_TOOLS_DIR=" + rebase_path("$root_out_dir/bin", dir),
     "CLANG_VENDOR_UTI=org.llvm.clang",