[Support/FileSystem] Introduce llvm::sys::fs::create_symbolic_link().

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203136 91177308-0d34-0410-b5e6-96231b3b80d8
[Support/LockFileManager] Use symbolic link for the lock file.

Hard links do not work on SMB network directories, and it causes us to fail
to build clang module files if the module cache is in such a directory.
rdar://15944959

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203137 91177308-0d34-0410-b5e6-96231b3b80d8