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[clangd] Define a compact binary serialization fomat for symbol slab/index.

Description

[clangd] Define a compact binary serialization fomat for symbol slab/index.

Summary:
This is intended to replace the current YAML format for general use.
It's ~10x more compact than YAML, and ~40% more compact than gzipped YAML:

llvmidx.riff = 20M, llvmidx.yaml = 272M, llvmidx.yaml.gz = 32M

It's also simpler/faster to read and write.

The format is a RIFF container (chunks of (type, size, data)) with:

  • a compressed string table
  • simple binary encoding of symbols (with varints for compactness)

It can be extended to include occurrences, Dex posting lists, etc.

There's no rich backwards-compatibility scheme, but a version number is included
so we can detect incompatible files and do ad-hoc back-compat.

Alternatives considered:

  • compressed YAML or JSON: bulky and slow to load
  • llvm bitstream: confusing model and libraries are hard to use. My attempt produced slightly larger files, and the code was longer and slower.
  • protobuf or similar: would be really nice (esp for back-compat) but the dependency is a big hassle
  • ad-hoc binary format without a container: it seems clear we're going to add posting lists and occurrences here, and that they will benefit from sharing a string table. The container makes it easy to debug these pieces in isolation, and make them optional.

Reviewers: ioeric

Subscribers: mgorny, ilya-biryukov, MaskRay, jkorous, mgrang, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D51585

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