…ed as used when checking for unused qualifications in resolve_path (fixes rust-lang#30078)

Turns out that calling `resolve_type_variables_if_possible` in a O(n^2)
loop is a bad idea. Now we just resolve each copy of the region variable
to its lowest name each time (we resolve the region variable to its lowest
name, rather than to its unify-table name to avoid the risk of
the unify-table name changing infinitely many times. That may be
not a problem in practice, but I am not sure of it).

We can now handle name resolution errors and get past type checking (if we're a bit lucky). This is the first step towards doing code completion for partial programs (we need error recovery in the parser and early access to save-analysis).

This fixes a bug in which unused imports can get wrongly marked as used when checking for unused qualifications in `resolve_path` (issue rust-lang#30078), and it removes unused imports that were previously undetected because of the bug.

…omatsakis

Turns out that calling `resolve_type_variables_if_possible` in a O(n^2)
loop is a bad idea. Now we just resolve each copy of the region variable
to its lowest name each time (we resolve the region variable to its lowest
name, rather than to its unify-table name to avoid the risk of
the unify-table name changing infinitely many times. That may be
not a problem in practice, but I am not sure of it).

…hton

The BtreeSet main description mentions `BTreeMap`, `Ord`, `Cell`, `RefCell`.

This patch adds links to their documentation.

…r=nrc

r? @nrc

…chton

I think we can close rust-lang#26873 with these tests.