Delete the setModule method from the Linker.

Delete the setModule method from the Linker.

It was only used from LTO for a debug feature, and LTO can just create
another linker.

It is pretty odd to have a method to reset the module in the middle of a
link. It would make IdentifiedStructTypes inconsistent with the Module
for example.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254434 91177308-0d34-0410-b5e6-96231b3b80d8