[clangd] Switch from YAMLParser to JSONExpr

Summary:
 - Converted Protocol.h parse() functions to take JSON::Expr.
   These no longer detect and log unknown fields, as this is not that
   useful and no longer free.
   I haven't changed the error handling too much: fields that were
   treated as optional before are still optional, even when it's wrong.
   Exception: object properties with the wrong type are now ignored.
 - Made JSONRPCDispatcher parse using json::parse
 - The bug where 'method' must come before 'params' in the stream is
 fixed as a side-effect. (And the same bug in executeCommand).
 - Some parser crashers fixed as a side effect.
   e.g. https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=3890
 - The debug stream now prettyprints the input messages with --pretty.
 - Request params are attached to traces when tracing is enabled.
 - Fixed some bugs in tests (errors tolerated by YAMLParser, and
 off-by-ones in Content-Length that our null-termination was masking)
 - Fixed a random double-escape bug in ClangdLSPServer (it was our last
 use of YAMLParser!)

Reviewers: ilya-biryukov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D40406

llvm-svn: 319159