[clangd] Extend SymbolOrigin, stop serializing it

New values:
- Split Dynamic into Open/Preamble
- Add Background (previously was just Unknown)
- Soon: stdlib index

This requires extending to 16 bits, which fits within the padding of Symbol.
Unfortunately we're also *serializing* SymbolOrigin as a fixed 8 bits.

Stop serializing SymbolOrigin:
- conceptually, the source is whoever indexes or *deserializes* a symbol
- deserialization takes SymbolOrigin as a parameter and stamps it on each sym
- this is a breaking format change

Differential Revision: https://reviews.llvm.org/D115243