Add a 'deleteModule' method to the Linker class.

This deletes the Module ivar instead of having the LTO code generater do it. It
also sets the pointer to 'NULL', so that if it's used again it will abort
quickly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192778 91177308-0d34-0410-b5e6-96231b3b80d8