Provide a fix-it that inserts omitted generic parameters when they can't be inferred.

[jrose-apple]

• Explanation: If someone tries to use a generic type without providing generic arguments, they'll get an error about how the generic parameters couldn't be inferred. In these cases, the most helpful thing to do is insert reasonable default types or placeholders. Provide a note with a fix-it, and auto-apply that fix-it during migration.
• Scope: Affects failure diagnosis for generic types without generic arguments, in both type and expression position.
• Issue: rdar://problem/27087345
• Reviewed by: @nkcsgexi
• Risk: Medium-low. Only fires during error diagnosis, but it is looking up a fair amount of information to do its work, which could mean exposing latent bugs. Most likely, though, the worst that will happen is it generates a bogus fix-it at some point.
• Testing: Added compiler regression tests, verified that the originally reported case now gets useful fix-its.

[jrose-apple]

@swift-ci Please test