[ADT] Fix `utohexstr` of zero with fixed width

[IllogicalMagic]

Summary

Fixed-width implementation of utohexstr contained a bug where extra zero was added when width argument is present and converted value is zero.

Add check for zero width in zero-handling path to fix the issue. Add unit tests for zero input.

llvm/include/llvm/ADT/StringExtras.h

inline char toUpper(char x) {
   return x;
 }
 
 inline std::string utohexstr(uint64_t X, bool LowerCase = false,
                              unsigned Width = 0) {
   char Buffer[17];
   char *BufPtr = std::end(Buffer);
 
-  if (X == 0) *--BufPtr = '0';
+  if (X == 0 && Width == 0)
+    *--BufPtr = '0';
 
   for (unsigned i = 0; Width ? (i < Width) : X; ++i) {
     unsigned char Mod = static_cast<unsigned char>(X) & 15;
     *--BufPtr = hexdigit(Mod, LowerCase);
     X >>= 4;
   }
 
   return std::string(BufPtr, std::end(Buffer));

llvm/unittests/ADT/StringExtrasTest.cpp

TEST(StringExtrasTest, ToAndFromHex) {
   std::string IgnoredOutput;
   EXPECT_FALSE(tryGetFromHex(InvalidStr, IgnoredOutput));
 }
 
 TEST(StringExtrasTest, UINT64ToHex) {
   EXPECT_EQ(utohexstr(0xA0u), "A0");
   EXPECT_EQ(utohexstr(0xA0u, false, 4), "00A0");
   EXPECT_EQ(utohexstr(0xA0u, false, 8), "000000A0");
+  EXPECT_EQ(utohexstr(0x0u), "0");
+  EXPECT_EQ(utohexstr(0x0u, false, 4), "0000");
 }
 
 TEST(StringExtrasTest, to_float) {
   float F;
   EXPECT_TRUE(to_float("4.7", F));
   EXPECT_FLOAT_EQ(4.7f, F);
 
   double D;