[Runtime] NFC: Move LLVM_ATTRIBUTE_USED from declaration to definition

This is both more correct and now required to keep building with
top-of-tree clang.

stdlib/public/runtime/ProtocolConformance.cpp

@@ -31,7 +31,9 @@
 using namespace swift;
 
 #ifndef NDEBUG
-template <> void ProtocolDescriptor::dump() const {
+template <>
+LLVM_ATTRIBUTE_USED
+void ProtocolDescriptor::dump() const {
   printf("TargetProtocolDescriptor.\n"
          "Name: \"%s\".\n",
          Name.get());
@@ -93,7 +95,9 @@ template<> void ProtocolConformanceDescriptor::dump() const {
 #endif
 
 #ifndef NDEBUG
-template<> void ProtocolConformanceDescriptor::verify() const {
+template<>
+LLVM_ATTRIBUTE_USED
+void ProtocolConformanceDescriptor::verify() const {
   auto typeKind = unsigned(getTypeKind());
   assert(((unsigned(TypeReferenceKind::First_Kind) <= typeKind) &&
           (unsigned(TypeReferenceKind::Last_Kind) >= typeKind)) &&