[clang] [clang][serialization] Blobify IMPORTS strings and signatures (PR #116095)

[Ben Langmuir via cfe-commits]

https://github.com/benlangmuir approved this pull request.

Gotta love having three copies of this code 😬 but obviously not something you need to fix here.  LGTM

https://github.com/llvm/llvm-project/pull/116095