[mlir][LLVM] Stop importing module location for all unknown locs

This commit changes the LLVM IR import to use UnkownLoc for missing
debug locations. This change ensures that we do not accidentially
introduce faulty locations that can influence debugging post export.

This behavior change is not applied to locations of global metadata
operations, as their location will not be exported.

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D154416