const char* for LLVMTargetMachineEmitToFile's argument

The `LLVMTargetMachineEmitToFile` takes a `char* Filename` right now, but it doesn't modify it.
This is annoying to use in the case where you want to pass a const string, because you either have to remove the const, or copy it somewhere else and pass that. Either way, it's not very nice.

I added a const and clang formatted it. This shouldn't break any ABI in my opinion.
I'm sorry but I didn't know whom to put as reviewer for this, so I chose someone with a lot of commits from the .cpp file.

Reviewed By: deadalnix

Differential Revision: https://reviews.llvm.org/D124453