[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