Fix compile error in UnresolvedSetTest.cpp, again

My previous fix used a gcc-style attribute, but not all compilers will accept
that. Instead use [[maybe_unused]], which is what we use elsewhere for this
kind of thing.

clang/unittests/AST/UnresolvedSetTest.cpp

@@ -5,7 +5,7 @@ namespace clang {
 class NamedDecl {
   // DeclAccessPair assumes that NamedDecl is at least 4-byte aligned, so we
   // we need to have a dummy value to make this dummy NamedDecl also be aligned.
-  int dummy __attribute__((unused));
+  [[maybe_unused]] int dummy;
 
 public:
   NamedDecl() {}