[mlir][llvm] Import additional debug info from LLVMIR.

Add a DebugImporter to convert LLVMIR debug metadata into
MLIR debug attributes. It is the counterpart to the
DebugTranslation class and supports the same attributes.
The revision only supports the translation of instruction,
function, and module debug information. The import of
intrinsics is left to a later revision.

Depends on D138206

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D138209