Bug 1358169: Fix Span.h constexprs for VS2017. r=froydnj

I can't speak for whether the MOZ_SPAN_GCC_CONSTEXPR on operator> is necessary, but I'm going on the assumption that any callers of it must also be in the same bucket of constexpr-ness.

mfbt/Span.h

@@ -284,20 +284,20 @@ public:
     return lhs.index_ < rhs.index_;
   }
 
-  constexpr friend bool operator<=(const span_iterator& lhs,
-                                   const span_iterator& rhs)
+  MOZ_SPAN_GCC_CONSTEXPR friend bool operator<=(const span_iterator& lhs,
+                                                const span_iterator& rhs)
   {
     return !(rhs < lhs);
   }
 
-  constexpr friend bool operator>(const span_iterator& lhs,
-                                  const span_iterator& rhs)
+  MOZ_SPAN_GCC_CONSTEXPR friend bool operator>(const span_iterator& lhs,
+                                               const span_iterator& rhs)
   {
     return rhs < lhs;
   }
 
-  constexpr friend bool operator>=(const span_iterator& lhs,
-                                   const span_iterator& rhs)
+  MOZ_SPAN_GCC_CONSTEXPR friend bool operator>=(const span_iterator& lhs,
+                                                const span_iterator& rhs)
   {
     return !(rhs > lhs);
   }