[clangd] Simplify client capabilities parsing.
Summary:
Instead of parsing into structs that mirror LSP, simply parse into a flat struct
that contains the info we need.
This is an exception to our strategy with Protocol.h, which seems justified:
- the structure here is very large and deeply nested
- we care about almost none of it
- we should never have to serialize client capabilities
Reviewers: kadircet
Subscribers: ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, cfe-commits
Differential Revision: https://reviews.llvm.org/D53266
llvm-svn: 344673