NameTypes and TypeSSA : Prefer _ over $ in names, and lint away _N suffixes (#5968)

Apparently $N (e.g. FooClass$5) is a convention in Java for anonymous classes, so our
$N that we use to disambiguate could be confusing. As the way we disambiguate does
not matter, switch to using _N. This PR does that in both TypeSSA and NameTypes.

Also make NameTypes "lint" names as it goes. That pass tries to give types nice names,
leaving existing ones that seem ok, and renaming long or unnamed ones. This PR makes
it aware of the _N notation and it tries to remove it, if removing it does not cause a
collision. An example of how that helps is if TypeSSA creates a subtype $Foo_0 and then
we manage to remove $Foo, then we can use the shorter name for the subtype.

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