Fix constexpr pre-c++20

Author: mborland

include/boost/int128/numeric.hpp

@@ -272,7 +272,9 @@ constexpr uint128_t gcd(uint128_t a, uint128_t b) noexcept
 
         if (a > b)
         {
-            std::swap(a, b);
+            const uint128_t temp {a};
+            a = b;
+            b = temp;
         }
 
         b -= a;

test/test_gcd_lcm.cpp

@@ -137,7 +137,7 @@ void test_negative_value()
 
     constexpr int128_t p {2 * 2 * 3};
     constexpr int128_t q {2 * 3 * 3};
-    static_assert(2 * 3 == gcd(p, q));
+    static_assert(2 * 3 == gcd(p, q), "Wrong");
 
     static_assert(gcd(int128_t{6},  int128_t{10}) == 2, "Wrong");
     static_assert(gcd(int128_t{6}, int128_t{-10}) == 2, "Wrong");