[Serialization] Remove `ORIGINAL_PCH_DIR` record

Use of `ORIGINAL_PCH_DIR` record has been superseeded by making PCH/PCM files with relocatable paths at write time.
Removing this record is useful for producing an output-path-independent PCH file and enable sharing of the same PCH file even
when it was intended for a different output path.

Differential Revision: https://reviews.llvm.org/D131124