[mlir][llvm] Move LLVM IR import into separate file (NFC).

The revision renames the Importer to ModuleImport and moves the class
out of the ConvertFromLLVMIR.cpp file into ModuleImport.h and
ModuleImport.cpp. Additionally, it introduces two helper methods
on the ModuleImport class that convert functions and globals, and it
merges the translateLLVMIRToModule function into the
registerFromLLVMIRTranslation function to match the design of
the MLIR to LLVM IR export.

This restructuring is a step towards the file structure of the export
and a preparation for an extensible import
https://discourse.llvm.org/t/rfc-extensible-llvm-ir-import/67256/6 that
uses a dialect interface to import intrinsics and metadata.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D140285