[mlir] support recursive type conversion of named LLVM structs

A previous commit added support for converting elemental types contained in
LLVM dialect types in case they were not compatible with the LLVM dialect. It
was missing support for named structs as they could be recursive, which was not
supported by the conversion infra. Now that it is, add support for converting
such named structs.

Depends On D113579

Reviewed By: wsmoses

Differential Revision: https://reviews.llvm.org/D113580