The commit adds serialization and de-serialization implementations for
the stored regions. Basically, the serialized representation of the
regions of a PP is a (ordered) sequence of source location encodings.
For de-serialization, regions from loaded files are stored by their ASTs.
When later one queries if a loaded location L is in an opt-out
region, PP looks up the regions of the loaded AST where L is at.
(Background if helps: a pair of `#pragma clang unsafe_buffer_usage begin/end` pragmas marks a
warning-opt-out region. The begin and end locations (opt-out regions)
are stored in preprocessor instances (PP) and will be queried by the
`-Wunsafe-buffer-usage` analyzer.)
The reported issue at upstream: https://github.com/llvm/llvm-project/issues/90501
rdar://124035402
