Parse fully-qualified associated types in generics without whitespace

[ftxqxd]

This modifies Parser::eat_lt to always split up <<s, instead of doing so only when a lifetime name followed or the force parameter (now removed) was true. This is because Foo<<TYPE is now a valid start to a type, whereas previously only Foo<<LIFETIME was valid.

This is a [breaking-change]. Change code that looks like this:

let x = foo asbar << 13;

to use parentheses, like this:

let x = (foo asbar) << 13;

Closes#17362.

[nikomatsakis]

I took the liberty of editing the PR message to include the all important "[breaking-change]" tag.

[nikomatsakis]

Oh, I see it's in the commit. Oh well.

[nikomatsakis]

I wonder if there is a way for us to give a nicer error message when things go wrong.

[ftxqxd]

I’ve thought previously about some kind of process after/during tokenisation (but before parsing) that would look for certain patterns of tokens that look like they could be this sort of subtle syntactical problem; e.g., in this case it would look for a sequence like as [short sequence of tokens] <<. It would print a message later if there was a parsing error (or maybe any kind of error) during compilation, but otherwise be silent. That sort of thing could perhaps also detect things like foo<int>() as well.