Move libc++ configs and flags out of //build/config/sanitizers

build/config/c++/c++.gni

+# Copyright 2017 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//build/config/sanitizers/sanitizers.gni")
+
+if (current_toolchain != default_toolchain) {
+  use_custom_libcxx = false
+}
+
+declare_args() {
+  # Use libc++ (buildtools/third_party/libc++ and
+  # buildtools/third_party/libc++abi) instead of stdlibc++ as standard
+  # library.
+  use_custom_libcxx =
+      (is_asan && is_linux && !is_chromeos &&
+       (!is_chromecast || is_cast_desktop_build)) || is_tsan || is_msan ||
+      is_ubsan || is_ubsan_security || (use_libfuzzer && !is_mac) || use_afl
+}