Allow llvm_find_program to find alternate names

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185859 91177308-0d34-0410-b5e6-96231b3b80d8
Find xdot or xdot.py.

Ubuntu installs this as xdot, so finding xdot.py would fail.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185860 91177308-0d34-0410-b5e6-96231b3b80d8