[LTO] Make getImportType a proper function (NFC) (#106450)
I'm planning to reduce the memory footprint of ThinLTO indexing by
changing ImportMapTy. A look-up of the import type will involve data
private to ImportMapTy, so it must be done by a member function of
ImportMapTy. This patch turns getImportType into a member function so
that a subsequent "real" change will just have to update the
implementation of the function in place.
