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

build/config/libc++/libc++.gni

+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_tsan || is_msan || is_ubsan ||
+      is_ubsan_security || (use_libfuzzer && !is_mac) || use_afl
+}