Support u8 as char8_t in StringPiece literals

Summary: In C++20, u8 prefixed strings are typed as `char8_t` not `char`, so we need to support that in `StringPiece`.

Reviewed By: yfeldblum

Differential Revision: D19822967

fbshipit-source-id: c3441ba718b40f0781766d4a80c3432b4514a35c

folly/Range.h

@@ -1523,6 +1523,14 @@ constexpr Range<char const*> operator"" _sp(
   return Range<char const*>(str, len);
 }
 
+#if __cpp_char8_t >= 201811L
+constexpr Range<char8_t const*> operator"" _sp(
+    char8_t const* str,
+    size_t len) noexcept {
+  return Range<char8_t const*>(str, len);
+}
+#endif
+
 constexpr Range<char16_t const*> operator"" _sp(
     char16_t const* str,
     size_t len) noexcept {

folly/test/RangeTest.cpp

@@ -1430,7 +1430,11 @@ TEST(Range, LiteralSuffix) {
   constexpr StringPiece piece = "hello";
   EXPECT_EQ(literalPiece, piece);
   constexpr auto literalPiece8 = u8"hello"_sp;
+#if __cpp_char8_t >= 201811L
+  constexpr Range<char8_t const*> piece8 = u8"hello";
+#else
   constexpr Range<char const*> piece8 = u8"hello";
+#endif
   EXPECT_EQ(literalPiece8, piece8);
   constexpr auto literalPiece16 = u"hello"_sp;
   constexpr Range<char16_t const*> piece16{u"hello", 5};