[clang] [clang][AST][NFC] Remove FIXME of `getQualifiedNameAsString()` (PR #218264)

[Baranov Victor via cfe-commits]

https://github.com/vbvictor approved this pull request.

LGTM, but please wait for some from clang side to approve as well (AaronBallman, cor3ntin, etc...)

https://github.com/llvm/llvm-project/pull/218264