[clang] [clang][ASTImporter] Fix possible crash "given incorrect InsertPos for specialization". (PR #89887)

[Donát Nagy via cfe-commits]

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

This seems to be a reasonable and straightforward improvement which rules out a hard-to test corner case. I think it's acceptable to merge this as it is now, because crafting a testcase would require a disproportionate amount of work.

https://github.com/llvm/llvm-project/pull/89887