[LTO] Update splitCodeGen to take a reference to the module. (NFC)

splitCodeGen does not need to take ownership of the module, as it
currently clones the original module for each split operation.

There is an ~4 year old fixme to change that, but until this is
addressed, the function can just take a reference to the module.

This makes the transition of LTOCodeGenerator to use LTOBackend a bit
easier, because under some circumstances, LTOCodeGenerator needs to
write the original module back after codegen.

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D95222