Fix compilation error with CATCH_CONFIG_GLOBAL_NEXTAFTER

Wrong nesting of namespaces resulted in the `Catch` namespace
being ambigous between `::Catch` and `::{anon}::Catch` namespaces.
This should fix it.

Closes #1761

include/internal/catch_matchers_floating.cpp

@@ -22,16 +22,6 @@
 
 
 namespace Catch {
-namespace Matchers {
-namespace Floating {
-enum class FloatingPointKind : uint8_t {
-    Float,
-    Double
-};
-}
-}
-}
-
 namespace {
 
     int32_t convert(float f) {
@@ -67,10 +57,16 @@ bool almostEqualUlps(FP lhs, FP rhs, uint64_t maxUlpDiff) {
         auto ulpDiff = std::abs(lc - rc);
         return static_cast<uint64_t>(ulpDiff) <= maxUlpDiff;
     }
+}
 
 #if defined(CATCH_CONFIG_GLOBAL_NEXTAFTER)
 
-namespace Catch {
+#if defined(__clang__)
+#pragma clang diagnostic push
+// The long double overload is currently unused
+#pragma clang diagnostic ignored "-Wunused-function"
+#endif
+
     float nextafter(float x, float y) {
         return ::nextafterf(x, y);
     }
@@ -82,10 +78,15 @@ namespace Catch {
     long double nextafter(long double x, long double y) {
         return ::nextafterl(x, y);
     }
-} // end namespace Catch
 
+#if defined(__clang__)
+#pragma clang diagnostic pop
 #endif
 
+#endif // ^^^ CATCH_CONFIG_GLOBAL_NEXTAFTER ^^^
+
+namespace {
+
 template <typename FP>
 FP step(FP start, FP direction, uint64_t steps) {
     for (uint64_t i = 0; i < steps; ++i) {
@@ -97,13 +98,17 @@ FP step(FP start, FP direction, uint64_t steps) {
     }
     return start;
 }
-
 } // end anonymous namespace
 
-
-namespace Catch {
 namespace Matchers {
 namespace Floating {
+
+    enum class FloatingPointKind : uint8_t {
+        Float,
+        Double
+    };
+
+
     WithinAbsMatcher::WithinAbsMatcher(double target, double margin)
         :m_target{ target }, m_margin{ margin } {
         CATCH_ENFORCE(margin >= 0, "Invalid margin: " << margin << '.'