[MLIR][LLVM] Add import flag to skip traversal of DICompositType's elems

This commit introduces a flag to allow skipping the potentially
recursive import of DICompositType elements. This patch is essentially a
bandaid for the still broken recursive debug type import.

Some of our downstream inputs are produced by excessive usage of
template meta programming, and thus contain tens of thousands of types
that all participate in such recursions. Unfortunately, the series of
patches that introduces type support is not easily revertable due to
being around for a while now and Modular depending on it.