[mlir][LLVMIR] Do not create pseudo debug file name using llvm::Instruction

Previously in mlir-translate, if debug info was absent in a
llvm::Instruction, we tried to create one using the name of its defined
value in a textual LLVM IR file as the (pseudo) debug file name.
However, in order to get that name, we need to call out to LLVM's
SlotTracker, which, surprisingly, took a lot of time. Judging from
the usefulness of such pseudo debug file name and the performance penalty
during translation, this patch simply use "imported-bitcode" as the
debug file name in these case. Eliminating the need of using (expensive)
LLVM value numbering.

Differential Revision: https://reviews.llvm.org/D134305