Auto merge of #62705 - petrochenkov:oh-bye-mark, r=matthewjasper

Auto merge of #62705 - petrochenkov:oh-bye-mark, r=matthewjasper

libsyntax: Rename `Mark` into `ExpnId`

"`Mark`" is an ID that identifies both a macro invocation (`foo!()`), and expansion process, and expansion result of that macro invocation.
The problem is that it's pretty hard to infer that from its name.

This PR renames it into `ExpnId` reflecting its meaning in most contexts.
(The contexts where it's meaning is closer to "macro invocation ID" are rarer.)

I've kept "mark" in the names of functions that add or remove marks to/from syntactic contexts, those marks are not just expansion IDs, but something more complex.