[llvm] [Support] Fix the style of "using" directives (NFC) (PR #157141)

[Yanzuo Liu via llvm-commits]

zwuis wrote:

IIUC you meant type alias.

https://cppreference.com/w/cpp/language/type_alias.html  
https://cppreference.com/w/cpp/language/namespace.html#Using-directives

https://github.com/llvm/llvm-project/pull/157141