[libc++] Extract std::fprintf into a function for use within the test suite

[ldionne]

Summary

This provides a single place for downstream to customize (or turn off)
printing information to stderr within the test suite.

libcxx/test/support/assert_macros.h

 //   the expected output, and the format string used. These macros allow
 //   logging additional arguments.
 
 #include "test_macros.h"
 
 #include <cstdio>
 #include <cstdlib>
 
+// This function prints the given arguments to standard error.
+//
+// Keeping this as a separate function is important since it provides a single point for
+// downstreams to customize how errors are printed on exotic targets, if needed.
+template <class ...Args>
+void test_eprintf(char const* fmt, Args const& ...args) {
+  std::fprintf(stderr, fmt, args...);
+}
+
 void test_log(const char* condition, const char* file, int line, const char* message) {
   const char* msg = condition ? "Assertion failure: " : "Unconditional failure:";
-  std::fprintf(stderr, "%s%s %s %d\n%s", msg, condition, file, line, message);
+  test_eprintf("%s%s %s %d\n%s", msg, condition, file, line, message);
 }
 
 template <class F>
 void test_log(const char* condition, const char* file, int line, const F& functor) {
-  std::fprintf(stderr, "Assertion failure: %s %s %d\n", condition, file, line);
+  test_eprintf("Assertion failure: %s %s %d\n", condition, file, line);
   functor();
 }
 
 template <class Arg>
 [[noreturn]] void test_fail(const char* file, int line, const Arg& arg) {
   test_log("", file, line, arg);
   std::abort();
 }

libcxx/test/support/concat_macros.h

 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef TEST_SUPPORT_CONCAT_MACROS_H
 #define TEST_SUPPORT_CONCAT_MACROS_H
 
 #include <cstdio>
 #include <string>
 
+#include "assert_macros.h"
 #include "test_macros.h"
 
 #ifndef TEST_HAS_NO_LOCALIZATION
 #  include <sstream>
 #endif
 
 #if TEST_STD_VER > 17
 
if constexpr ((test_char_streamable<Args> && ...)) {
     ((sstr << std::forward<Args>(args)), ...);
     return sstr.str();
   } else
 #  endif
     return "Message discarded since it can't be streamed to std::cerr.\n";
 }
 
 // Writes its arguments to stderr, using the test_concat_message helper.
-#  define TEST_WRITE_CONCATENATED(...) [&] { ::std::fprintf(stderr, "%s", ::test_concat_message(__VA_ARGS__).c_str()); }
+#  define TEST_WRITE_CONCATENATED(...) [&] { ::test_eprintf("%s", ::test_concat_message(__VA_ARGS__).c_str()); }
 
 #endif // TEST_STD_VER > 17
 
 #endif //  TEST_SUPPORT_CONCAT_MACROS_H