[LTO API] add lto_codegen_set_module to help ld64 debug LTO issues

[Manman Ren]

Hi Rafael and others,

When debugging LTO issues with ld64, I sometimes use -save-temps to save the merged optimized bitcode file, then invoke ld64 again on the single bitcode file.

I found out that llvm linking a single bitcode file via lto_codegen_add_module will generate a different bitcode file from the single input.
This makes debugging hard. With the newly-added lto_codegen_set_module, we can make sure the destination module is the same as the input.

The patch is attached. Let me know your thoughts,

Thanks,
Manman

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