[clang] [clang][ASTImporter] fix variable inline of CXX17 (PR #87314)

[Balazs Benics via cfe-commits]

https://github.com/steakhal requested changes to this pull request.

I only had generic nits, as I'm not really in the domain. Consider my review as such.

https://github.com/llvm/llvm-project/pull/87314