Hadrian: Delete target symlink in createFileLinkUntracked
Hadrian: Delete target symlink in createFileLinkUntracked

Previously createFileLinkUntracked would fail if the symlink already
existed.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc