Make operator<=> static

Apparently this counts as "namespace scope" (as opposed to an anonymous
namespace), and it still won't be visible outside the translation unit.

ConflictSet.cpp

@@ -8,7 +8,7 @@
 
 using Key = ConflictSet::Key;
 
-auto operator<=>(const Key &lhs, const Key &rhs) {
+static auto operator<=>(const Key &lhs, const Key &rhs) {
   const int minLen = std::min(lhs.len, rhs.len);
   const int c = memcmp(lhs.p, rhs.p, minLen);
   return c != 0 ? c <=> 0 : lhs.len <=> rhs.len;