[libc] Use the new wide integer to hex string facility in LibcTest.

[sivachandra]

Summary

The old code, which has regressed over many cleanups, has been replaced
with the new wide integer to hex string facility.

[michaelrj]

LGTM

libc/test/UnitTest/LibcTest.cpp

 #include "src/__support/UInt128.h"
 #include "test/UnitTest/TestLogger.h"
 
 namespace __llvm_libc {
 namespace testing {
 
 namespace internal {
 
-// When the value is UInt128 or __uint128_t, show its hexadecimal digits.
-// We cannot just use a UInt128 specialization as that resolves to only
-// one type, UInt<128> or __uint128_t. We want both overloads as we want to
-// be able to unittest UInt<128> on platforms where UInt128 resolves to
-// UInt128.
+// When the value is UInt128, __uint128_t or wider, show its hexadecimal digits.
 template <typename T>
-cpp::enable_if_t<cpp::is_integral_v<T> && cpp::is_unsigned_v<T>, cpp::string>
-describeValueUInt(T Value) {
+cpp::enable_if_t<cpp::is_integral_v<T> && cpp::is_unsigned_v<T> &&
+                     (sizeof(T) > sizeof(uint64_t)),
+                 cpp::string>
+describeValue(T Value) {
   static_assert(sizeof(T) % 8 == 0, "Unsupported size of UInt");
-  cpp::string S(sizeof(T) * 2, '0');
-
-  constexpr char HEXADECIMALS[16] = {'0', '1', '2', '3', '4', '5', '6', '7',
-                                     '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};
-  const size_t Size = S.size();
-  for (size_t I = 0; I < Size; I += 2, Value >>= 8) {
-    unsigned char Mod = static_cast<unsigned char>(Value) & 0xFF;
-    S[Size - I] = HEXADECIMALS[Mod & 0x0F];
-    S[Size - (I + 1)] = HEXADECIMALS[Mod & 0x0F];
-  }
+  char buf[IntegerToString::hex_bufsize<T>()];
+  IntegerToString::hex(Value, buf, false);
+  return "0x" + cpp::string(buf, sizeof(buf));
 
-  return "0x" + S;
-}
-
-// When the value is of integral type, just display it as normal.
+// When the value is of a standard integral type, just display it as normal.
 template <typename ValType>
-cpp::enable_if_t<cpp::is_integral_v<ValType>, cpp::string>
+cpp::enable_if_t<cpp::is_integral_v<ValType> &&
+                     sizeof(ValType) <= sizeof(uint64_t),
+                 cpp::string>
 describeValue(ValType Value) {
-  if constexpr (sizeof(ValType) <= sizeof(uint64_t)) {
-    return cpp::to_string(Value);
-  } else {
-    return describeValueUInt(Value);
-  }
 }
 
 cpp::string describeValue(cpp::string Value) { return Value; }
 cpp::string_view describeValue(cpp::string_view Value) { return Value; }
 
 template <typename ValType>
 void explainDifference(ValType LHS, ValType RHS, const char *LHSStr,
                        const char *RHSStr, const char *File, unsigned long Line,