Teach variance checker about the lifetime bounds that appear in trait object types.
auto merge of #18371 : nikomatsakis/rust/issue-18262, r=pcwalton

Teach variance checker about the lifetime bounds that appear in trait object types.

[breaking-change] This patch fixes a hole in the type system which resulted in lifetime parameters that were only used in trait objects not being checked. It's hard to characterize precisely the changes that might be needed to fix target code.

cc #18262 (this fixes the test case by @jakub- but I am not sure if this is the same issue that @alexcrichton was reporting)

r? @pnkfelix 

Fixes #18205