[libc++] Use proper shell escaping in the executors

This was originally committed as f8452ddf and reverted in 7cb1aa9d.
The issue was that shell builtins were being escaped too, and apparently
Bash won't execute a builtin when it is quoted e.g. '!'. Instead, it
thinks it's a command and it can't find it.

Re-committing the change with that issue fixed.