Use /usr/bin/env python instead of /usr/bin/python.

On operating systems like the BSDs, it is typically the case that
/usr/bin/python does not exist. We should therefore use /usr/bin/env
instead. This is also done in various other scripts in tools/.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216945 91177308-0d34-0410-b5e6-96231b3b80d8