Duplicate lint specifications are always bug!
Duplicate lint specifications are always bug!

Replace early_error and sess.err with bug!, in all cases. If the
compiler we're running with, including plugins, is registering something
twice, that's a (compiler/plugin) programmer error -- we should not try
to be nice at the cost of developer ergononomics (hiding the stacktrace
of the second registration is bad).

This also is basically a static bug in ~all cases so it should not be a
detriment to users, including with plugins.

graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust