Break up autolinking of generic types.

[reitzig]

Say we have code like this:

protocol Bar {}

class Foo<Bar> {}
class Barista: Bar {}

When we refer to Foo<Barista> in a documentation comment, we might expect Foo to link to Foo, and Barista to link to Barista (as is the case in declarations). Instead, the whole type links to Foo.