Auto merge of #130324 - petrochenkov:ctxtache, r=oli-obk

hygiene: Ensure uniqueness of `SyntaxContextData`s

`SyntaxContextData`s are basically interned with `SyntaxContext`s working as indices, so they are supposed to be unique.
However, currently duplicate `SyntaxContextData`s can be created during decoding from metadata or incremental cache.
This PR fixes that.

cc rust-lang/rust#129827 (comment)

Author: bors