Add a setting to not require --overwrite to overwrite commands.

Protecting against accidental overwriting of commands is good, but
having to pass a flag to overwrite the command when developing your
commands is pretty annoying.  This adds a setting to defeat the protection
so you can do this once at the start of your session and not have to
worry about it again.

Differential Revision: https://reviews.llvm.org/D122680