libutil: replace Pos operator< with <=> and constexpr ==

Change-Id: I60080c416b2ee2b0efce4584cf13f37c0596d73d

lix/libutil/position.cc

@@ -17,12 +17,6 @@ Pos::operator std::shared_ptr<Pos>() const
     return std::make_shared<Pos>(&*this);
 }
 
-bool Pos::operator<(const Pos &rhs) const
-{
-    return std::forward_as_tuple(line, column, origin)
-        < std::forward_as_tuple(rhs.line, rhs.column, rhs.origin);
-}
-
 std::optional<LinesOfCode> Pos::getCodeLines() const
 {
     if (line == 0)

lix/libutil/position.hh

@@ -67,9 +67,15 @@ struct Pos
 
     std::optional<LinesOfCode> getCodeLines() const;
 
-    bool operator==(const Pos & rhs) const = default;
-    bool operator!=(const Pos & rhs) const = default;
-    bool operator<(const Pos & rhs) const;
+    // Not defaulted because it's implicitly deleted because C++ is stupid.
+    constexpr friend auto operator<=>(Pos const & lhs, Pos const & rhs)
+    {
+        return std::forward_as_tuple(lhs.line, lhs.column, lhs.origin)
+            <=> std::forward_as_tuple(rhs.line, rhs.column, rhs.origin);
+    }
+
+    // operator<=> doesn't generate this because it's not defaulted.
+    constexpr friend bool operator==(Pos const & lhs, Pos const & rhs) = default;
 
     struct LinesIterator {
         using difference_type = size_t;