[clang] [clang] Speedup getFileIDLocal with a separate offset table. (PR #146604)

[Erich Keane via cfe-commits]

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

1 suggestion, else LGTM.  Please make sure aaron gets a chance to look if he wants.

https://github.com/llvm/llvm-project/pull/146604