[clang] serialize ORIGINAL_PCH_DIR relative to BaseDirectory

This diff changes the serialization of the `ORIGINAL_PCH_DIR`
entry in module files to be serialized relative to the module's
`BaseDirectory`. This will allow for the module to be relocatable
across machines.

The path is restored relative to the module's BaseDirectory on
deserialization.

Reviewed By: urnathan

Differential Revision: https://reviews.llvm.org/D124946