[libc] Implement perror (#143624)

The perror function writes an error message directly to stderr. This
patch adds an implementation, tests, and header generation details.

libc/test/src/stdio/CMakeLists.txt

@@ -357,6 +357,18 @@ add_libc_test(
     libc.src.stdio.puts
 )
 
+add_libc_test(
+  perror_test
+  HERMETIC_TEST_ONLY # writes to libc's stderr
+  SUITE
+    libc_stdio_unittests
+  SRCS
+    perror_test.cpp
+  DEPENDS
+    libc.src.stdio.perror
+    libc.src.errno.errno
+)
+
 add_libc_test(
   fputs_test
   HERMETIC_TEST_ONLY # writes to libc's stdout and stderr

libc/test/src/stdio/perror_test.cpp

@@ -0,0 +1,32 @@
+//===-- Unittests for perror ---------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/stdio/perror.h"
+
+#include "src/__support/libc_errno.h"
+#include "test/UnitTest/Test.h"
+
+// The standard says perror prints directly to stderr and returns nothing. This
+// makes it rather difficult to test automatically.
+
+// TODO: figure out redirecting stderr so this test can check correctness.
+TEST(LlvmLibcPerrorTest, PrintOut) {
+  LIBC_NAMESPACE::libc_errno = 0;
+  constexpr char simple[] = "A simple string";
+  LIBC_NAMESPACE::perror(simple);
+
+  // stick to stdc errno values, specifically 0, EDOM, ERANGE, and EILSEQ.
+  LIBC_NAMESPACE::libc_errno = EDOM;
+  LIBC_NAMESPACE::perror("Print this and an error");
+
+  LIBC_NAMESPACE::libc_errno = EILSEQ;
+  LIBC_NAMESPACE::perror("\0 shouldn't print this.");
+
+  LIBC_NAMESPACE::libc_errno = ERANGE;
+  LIBC_NAMESPACE::perror(nullptr);
+}