Rollup merge of #33576 - soltanmm:vtable, r=nikomatsakis

Plumb inference obligations through selection, take 2

Using a `SnapshotVec` and dumping inferred obligations into `Vtable` variants.

r? @nikomatsakis

diff --cc src/librustc/traits/project.rs
Simple merge

diff --cc src/librustc_trans/callee.rs
Simple merge