Auto merge of #24932 - pnkfelix:fix-issue-24687, r=huonw

metdata: Fix zero-normalization of the pos of a `MultiByteChar`

Fix #24687

The source byte/character mappings for every crate track the collection of multi-characters from its source files specially.  When we import the source information for another file into the current compilation unit, we assign its byte-positions unique values by shifting them all by a fixed adjustment, tracked in the `start_pos` field.  But when we pull out the source span information for one function from one crate and into our own crate, we need to re-normalize the byte positions: subtracting the old `start_pos` and adding the new `start_pos`. The `new_imported_filemap(..)` method handles adding the new `start_pos`, so all `creader` needs to do is re-normalize each `pos` to zero.

It seems like it was indeed trying to do this, but it mistakenly added the old `start_pos` instead of subtracting it.

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