[libc][nfc] add CPP Limits.h for numeric_limits

[michaelrj]

Summary

Add an implementation of numeric_limits for use in str_conv_utils.
It currently only supports the basic integer types, with more types
coming as needed.

[sivachandra]

libc/utils/CPP/Limits.h

16

Move this outside of the namespace?

18

Name the class NumericLimits to follow the convention of the rest of CPP utils.

20

Make these static methods constexpr also.

[michaelrj]

Rename numeric_limits to NumericLimits and make all the functions constexpr.

[sivachandra]

libc/utils/CPP/Limits.h

23

Add a TODO saying specializations will be added as needed.

[michaelrj]

add a comment with a todo

libc/utils/CPP/CMakeLists.txt

 add_header_library(
   standalone_cpp
   HDRS
     Array.h
     ArrayRef.h
     Bitset.h
     Functional.h
     StringView.h
     TypeTraits.h
+    Limits.h
 )

libc/utils/CPP/Limits.h

+//===-- A self contained equivalent of std::limits --------------*- C++ -*-===//
+//
+// 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 LLVM_LIBC_UTILS_CPP_LIMITS_H
+#define LLVM_LIBC_UTILS_CPP_LIMITS_H
+
+#include <limits.h>
+
+namespace __llvm_libc {
+namespace cpp {
+
+template <class T> class NumericLimits {
+public:
+  static constexpr T max();
+  static constexpr T min();
+};
+
+// TODO: Add NumericLimits specializations as needed for new types.
+
+template <> class NumericLimits<int> {
+public:
+  static constexpr int max() { return INT_MAX; }
+  static constexpr int min() { return INT_MIN; }
+};
+template <> class NumericLimits<unsigned int> {
+public:
+  static constexpr unsigned int max() { return UINT_MAX; }
+  static constexpr unsigned int min() { return 0; }
+};
+template <> class NumericLimits<long> {
+public:
+  static constexpr long max() { return LONG_MAX; }
+  static constexpr long min() { return LONG_MIN; }
+};
+template <> class NumericLimits<unsigned long> {
+public:
+  static constexpr long max() { return ULONG_MAX; }
+  static constexpr long min() { return 0; }
+};
+template <> class NumericLimits<long long> {
+public:
+  static constexpr long long max() { return LLONG_MAX; }
+  static constexpr long long min() { return LLONG_MIN; }
+};
+template <> class NumericLimits<unsigned long long> {
+public:
+  static constexpr long max() { return ULLONG_MAX; }
+  static constexpr long min() { return 0; }
+};
+
+} // namespace cpp
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_UTILS_CPP_LIMITS_H