-Wdeprecated-literal-operator fixes

Summary:
Fix deprecated literal operators:
```
./xplat/folly/FixedString.h:2923:1: error: identifier '_fs64' preceded by whitespace in a literal operator declaration is deprecated [-Werror,-Wdeprecated-literal-operator]
 2923 | FOLLY_DEFINE_FIXED_STRING_UDL(64)
      | ^
./xplat/folly/FixedString.h:2912:39: note: expanded from macro 'FOLLY_DEFINE_FIXED_STRING_UDL'
 2912 |   constexpr FixedString<N> operator"" _fs##N(                \
      |                                       ^
```

Reviewed By: drodriguez

Differential Revision: D57494889

fbshipit-source-id: 7120f44d28abdcc6b8cd4dfa86a78c6903d805ab

folly/FixedString.h

@@ -2900,7 +2900,7 @@ constexpr const std::size_t& npos = detail::fixedstring::FixedStringBase::npos;
  *   `FixedString<8>`, `FixedString<16>`, etc.
  */
 template <class Char, Char... Cs>
-constexpr BasicFixedString<Char, sizeof...(Cs)> operator"" _fs() noexcept {
+constexpr BasicFixedString<Char, sizeof...(Cs)> operator""_fs() noexcept {
   const Char a[] = {Cs..., Char(0)};
   return {+a, sizeof...(Cs)};
 }
@@ -2909,7 +2909,7 @@ constexpr BasicFixedString<Char, sizeof...(Cs)> operator"" _fs() noexcept {
 #endif
 
 #define FOLLY_DEFINE_FIXED_STRING_UDL(N)                     \
-  constexpr FixedString<N> operator"" _fs##N(                \
+  constexpr FixedString<N> operator""_fs##N(                 \
       const char* that, std::size_t count) noexcept(false) { \
     return {that, count};                                    \
   }                                                          \

folly/system/Shell.h

@@ -61,7 +61,7 @@ struct ShellCmdFormat {
 
 inline namespace literals {
 inline namespace shell_literals {
-constexpr detail::ShellCmdFormat operator"" _shellify(
+constexpr detail::ShellCmdFormat operator""_shellify(
     char const* name, std::size_t length) {
   return {folly::StringPiece(name, length)};
 }