Auto merge of #46425 - eddyb:mir-place, r=nikomatsakis

MIR: change "lvalue" terminology to "place".

As pointed out elsewhere, "lvalue" vs "rvalue" is a misleading/obscure distinction and several other choices have been proposed, the one I prefer being "place" vs "value".

This PR only touches the "lvalue" side, and only in MIR-related code, as it's already a lot and could rot.