Last batch of cleanups to Linker.h.

Update comments, fix * placement, fix method names that are not
used in clang, add a linkInModule that takes a Mode and put it
in Linker.cpp.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181099 91177308-0d34-0410-b5e6-96231b3b80d8