Don't avoid to load trait impl .class without inliner.

When the inliner is on, it is necessary to load class
files containing the implementation of traits, i.e.,
those ending in $class.class. This is needed to be
able to inline these methods. However it is useless
if the inliner is disabled.

The previous logic avoided to put these files on the
classpath when the inliner was off. However, this
complicates things, and it makes the classpath
mechanism dependent on something totally unrelated.

On this basis, this commit removes that logic, and
instead trait impl .class files are always kept on
the classpath.
Add -Yno-load-impl-class disabling loading of $class.class files.

The parent commit, a3f71badf67bbaac1a4ba16f68211ea6e31aa473,
removed some logic preventing $class.class files to be
loaded. It did so only when the inliner was off.

Should this cause any issue, this option provides a means to
restore the old behavior by *never* loading $class.class files.

So, using -inline -Yno-load-impl-class will not load
$class.class files either (where previously -inline would
load them). The two old behaviors being available by *either*
using -inline *or* -Yno-load-impl-class (not both).
Merge pull request #2977 from sjrd/topic/remove-classpath-logic-dependent-on-inline

Don't avoid to load trait impl .class without inliner.