[clangd] NFC: Prefer `isa<>` to `dyn_cast<>` to do the checking.

[MTC]

Summary

Prefer isa<> to dyn_cast<> when there only need a checking.

[MaskRay]

clangd/AST.cpp

98

isa<NamespaceDecl>(ND) also works.

[MTC]

clangd/AST.cpp

98

Thanks for the tips! I will update it latter.

[ilya-biryukov]

Ah, sorry, landed a cleanup fixing the warnings (rL347539) before seeing this change.
But given that isa<> are still better than dyn_cast<>, this change might still be worth landing.

[MTC]

Use more concise form.

[MTC]

In D54878#1307726, @ilya-biryukov wrote:

But given that isa<> are still better than dyn_cast<>, this change might still be worth landing.

We can land this change this time or do the cleaning job in other patches in the future, it's all up to you guys, the active clangd contributors :).

[ilya-biryukov]

In D54878#1307845, @MTC wrote:

We can land this change this time or do the cleaning job in other patches in the future, it's all up to you guys, the active clangd contributors :).

Please land this, it's a useful cleanup.
Unless you'd prefer to not spend any more time on this, in which case I'd just land it for you :-)

clangd/AST.cpp

std::string printName(const ASTContext &Ctx, const NamedDecl &ND) {
   ND.getDeclName().print(Out, PP);
   if (!Out.str().empty()) {
     // FIXME(ibiryukov): do not show args not explicitly written by the user.
     if (auto *ArgList = getTemplateSpecializationArgs(ND))
       printTemplateArgumentList(Out, ArgList->asArray(), PP);
     return Out.str();
   }
   // The name was empty, so present an anonymous entity.
-  if (llvm::dyn_cast<NamespaceDecl>(&ND))
+  if (isa<NamespaceDecl>(ND))
     return "(anonymous namespace)";
   if (auto *Cls = llvm::dyn_cast<RecordDecl>(&ND))
     return ("(anonymous " + Cls->getKindName() + ")").str();
-  if (llvm::dyn_cast<EnumDecl>(&ND))
+  if (isa<EnumDecl>(ND))
     return "(anonymous enum)";
   return "(anonymous)";
 }
 
 std::string printNamespaceScope(const DeclContext &DC) {
   for (const auto *Ctx = &DC; Ctx != nullptr; Ctx = Ctx->getParent())
     if (const auto *NS = dyn_cast<NamespaceDecl>(Ctx))
       if (!NS->isAnonymousNamespace() && !NS->isInlineNamespace())