[clang] [clang][modules] Adopt `FileEntryRef` in the `HeaderFileInfo` block in PCM files (PR #67383)

[Richard Howell via cfe-commits]

rmaz wrote:

LGTM, possible to come up with some test case with multiple virtual entries to the same file that would now be serialized once?

https://github.com/llvm/llvm-project/pull/67383