Cleanup the interface for creating soft or hard links.

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