Add a script to run a command but ignore its return code. This script

Add a script to run a command but ignore its return code. This script
always returns 0. This is useful with the llvm.exp based dejagnu testing
when a test wants to check the error output of tool invocation that
returns non-zero. Since every command is checked with llvm.exp, there needs
to be a way to prevent that checking and this script is it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@35991 91177308-0d34-0410-b5e6-96231b3b80d8