[clangd] Migrate away from std::nullopt (NFC) (#145841)

ArrayRef has a constructor that accepts std::nullopt.  This
constructor dates back to the days when we still had llvm::Optional.

Since the use of std::nullopt outside the context of std::optional is
kind of abuse and not intuitive to new comers, I would like to move
away from the constructor and eventually remove it.

This patch replaces std::nullopt with {}.

clang-tools-extra/clangd/AST.cpp

@@ -440,7 +440,7 @@ QualType declaredType(const TypeDecl *D) {
     if (const auto *Args = CTSD->getTemplateArgsAsWritten())
       return Context.getTemplateSpecializationType(
           TemplateName(CTSD->getSpecializedTemplate()), Args->arguments(),
-          /*CanonicalArgs=*/std::nullopt);
+          /*CanonicalArgs=*/{});
   return Context.getTypeDeclType(D);
 }