[AST] Use llvm::iterator_range::empty (NFC) (#151800)

With empty, we don't have to repeat the whole expression.

Author: kazutakahirata

clang/lib/AST/MicrosoftMangle.cpp

@@ -1298,8 +1298,7 @@ void MicrosoftCXXNameMangler::mangleUnqualifiedName(GlobalDecl GD,
         Name += "<unnamed-type-";
         Name += TND->getName();
       } else if (isa<EnumDecl>(TD) &&
-                 cast<EnumDecl>(TD)->enumerator_begin() !=
-                     cast<EnumDecl>(TD)->enumerator_end()) {
+                 !cast<EnumDecl>(TD)->enumerators().empty()) {
         // Anonymous non-empty enums mangle in the first enumerator.
         auto *ED = cast<EnumDecl>(TD);
         Name += "<unnamed-enum-";