[libc++][format] adds a basic fuzzer test. (#87883)

This adds an initial fuzzer. Different formatting arguments will execute
different code paths. This will be tested by different fuzzer tests.

The code is based on a sample provided by Louis.

Author: mordante

libcxx/test/libcxx/fuzzing/format_no_args.pass.cpp

@@ -0,0 +1,30 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: no-exceptions
+
+// UNSUPPORTED: GCC-ALWAYS_INLINE-FIXME
+
+// XFAIL: availability-fp_to_chars-missing
+
+#include <cstdint>
+#include <format>
+#include <string_view>
+
+#include "fuzz.h"
+
+extern "C" int LLVMFuzzerTestOneInput(const std::uint8_t* data, std::size_t size) {
+  try {
+    [[maybe_unused]] auto result = std::vformat(std::string_view{(const char*)(data), size}, std::make_format_args());
+  } catch (std::format_error const&) {
+    // If the fuzzing input isn't a valid thing we can format and we detect it, it's okay. We are looking for crashes.
+    return 0;
+  }
+  return 0;
+}