[mlir][llvm] Prioritize DILocalScope over file loc

This commit ensures that the LLVMIR export prioritizes existing
DILocalScope attribute information as location scopes over files
constructed from filenames. All DILocalScope attributes contain file
information, so no information is lost. The previous implementation
caused the introduction of superfluous DILexicalBlockFile nodes in
certain cases. The old implementation remains as a fallback when no
DILocalScope is present.

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D144968