Auto merge of #115082 - Zoxc:syntax-context-decode-race, r=cjgillot

Fix races conditions with `SyntaxContext` decoding

This changes `SyntaxContext` decoding to work with concurrent decoding. The `remapped_ctxts` field now only stores `SyntaxContext` which have completed decoding, while the new `decoding` and `local_in_progress` keeps track of `SyntaxContext`s which are in process of being decoding and on which threads.

This fixes 2 issues with the current implementation. It can return an `SyntaxContext` which contains dummy data if another thread starts decoding before the first one has completely finished. Multiple threads could also allocate multiple `SyntaxContext`s for the same `raw_id`.

Author: bors