Cleanup the interface for creating soft or hard links.

Before this patch the unix code for creating hardlinks was unused. The code
for creating symbolic links was implemented in lib/Support/LockFileManager.cpp
and the code for creating hard links in lib/Support/*/Path.inc.

The only use we have for these is in LockFileManager.cpp and it can use both
soft and hard links. Just have a create_link function that creates one or the
other depending on the platform.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203596 91177308-0d34-0410-b5e6-96231b3b80d8