[mlir][llvmir] Simpler error handling in ConvertFromLLVMIR (nfc).

The revision renames some methods of the Importer and changes
the error handling to be closer the ModuleTranslation. In particular,
processValue -> lookupValue and processType -> convertType
now fail if the translation fails (instead of returning an error),
which simplifies the error handling.

The revision prepares a follow up commit that will import
LLVMIR intrinsics using tablegen.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D135349

Author: gysit