Print the sign of negative infinity

[logan]

Summary

This commit makes llvm::write_double print -inf as "-INF" instead of "INF".

[sepavloff]

LGTM.

Thanks!

llvm/lib/Support/NativeFormatting.cpp

 void llvm::write_double(raw_ostream &S, double N, FloatStyle Style,
                         Optional<size_t> Precision) {
   size_t Prec = Precision.getValueOr(getDefaultPrecision(Style));
 
   if (std::isnan(N)) {
     S << "nan";
     return;
   } else if (std::isinf(N)) {
-    S << "INF";
+    S << (std::signbit(N) ? "-INF" : "INF");
     return;
   }
 
   char Letter;
   if (Style == FloatStyle::Exponent)
     Letter = 'e';
   else if (Style == FloatStyle::ExponentUpper)
     Letter = 'E';

llvm/unittests/Support/NativeFormatTests.cpp

TEST(NativeFormatTest, BoundaryTests) {
   EXPECT_EQ("4294967295", format_number(UINT32_MAX, IntegerStyle::Integer));
   EXPECT_EQ("2147483647", format_number(INT32_MAX, IntegerStyle::Integer));
   EXPECT_EQ("-2147483648", format_number(INT32_MIN, IntegerStyle::Integer));
 
   EXPECT_EQ("nan", format_number(std::numeric_limits<double>::quiet_NaN(),
                                  FloatStyle::Fixed));
   EXPECT_EQ("INF", format_number(std::numeric_limits<double>::infinity(),
                                  FloatStyle::Fixed));
+  EXPECT_EQ("-INF", format_number(-std::numeric_limits<double>::infinity(),
+                                  FloatStyle::Fixed));
 }
 
 TEST(NativeFormatTest, HexTests) {
   // Test hex formatting with different widths and precisions.
 
   // Width less than the value should print the full value anyway.
   EXPECT_EQ("0x0", format_number(0, HexPrintStyle::PrefixLower, 0));
   EXPECT_EQ("0xabcde", format_number(0xABCDE, HexPrintStyle::PrefixLower, 3));