[mlir][llvm] Drop cyclic dependencies during debug metadata import.

This revision fixes the import of LLVM IR to handle debug metadata with
cyclic dependencies. It deletes the elements list of the composite type
if a cyclic dependency is detected. The revision is meant as a band aid
to avoid infinite recursion during the import of cyclic debug metadata.
Long term solutions are currently discussed here:
https://discourse.llvm.org/t/handling-cyclic-dependencies-in-debug-info/67526/4

Reviewed By: Dinistro

Differential Revision: https://reviews.llvm.org/D142086