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

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

@aaron You introduced the fixme in 75ee4cc8302aa68464e55b273ebd53f85252268e - given the churn it would create and the opposition of tooling which uses that a lot, no objection to removing the fixme?

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