Fix the PrefixExecutor.

The PrefixExecutor wasn't passing the exe_path down the chain, so the
command was overriding that, the work_dir was being passed as the
command, and so on.

I've cleaned up a few pylint issues while I was here.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@231496 91177308-0d34-0410-b5e6-96231b3b80d8
Change-Id: Ice5baa25246cf5830c91b329eaeb819ae0dd1aad
1 file changed