[libc++] Move __clamp_to_integral to <cmath>, and harden against min()/max() macros

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@370900 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 87e6185621d332d2ef8695b94778663380c83bb9)

Bug: https://bugs.chromium.org/p/chromium/issues/detail?id=994957
Bug: http://b/139690488
Change-Id: Ice92a8ea6f675be069a70c5ad1003cf99b4fc474
(cherry picked from commit 82e0483c4a47233ed2f7bab545b32e5ddd474e59)

test/libcxx/numerics/c.math/undef_min_max.pass.cpp

@@ -0,0 +1,19 @@
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#if defined(__GNUC__) || defined(__clang__)
+#pragma GCC diagnostic ignored "-W#warnings"
+#endif
+
+#define min THIS IS A NASTY MACRO!
+#define max THIS IS A NASTY MACRO!
+
+#include <cmath>
+
+#include "test_macros.h"
+
+int main(int, char**) { return 0; }