[clang] [clang][Parser] Fix crash on malformed using declaration in constexpr function (PR #144286)

[Iris Shi via cfe-commits]

el-ev wrote:

> Next time please add more details in the summary, this is key for reviewers so that they can digest the PR without leaving for more context. Even in very simple PRs it is good to do.
> 
> For this one something along the lines of `Parser::ParseUsingDeclaration was not that the result of ParseAliasDeclarationAfterDeclarator was not a nullptr. The fix is to check the result and return nullptr`

Thank you for your kind suggestions.

https://github.com/llvm/llvm-project/pull/144286