[clangd] Enforce rules around "initialize" request, and create ClangdServer lazily.

Summary:
LSP is a slightly awkward map to C++ object lifetimes: the initialize request
is part of the protocol and provides information that doesn't change over the
lifetime of the server.

Until now, we handled this by initializing ClangdServer and ClangdLSPServer
right away, and making anything that can be set in the "initialize" request
mutable.
With this patch, we create ClangdLSPServer immediately, but defer creating
ClangdServer until "initialize". This opens the door to passing the relevant
initialize params in the constructor and storing them immutably.
(That change isn't actually done in this patch).

To make this safe, we have the MessageDispatcher enforce that the "initialize"
method is called before any other (as required by LSP). That way each method
handler can assume Server is initialized, as today.

As usual, while implementing this I found places where our test cases violated
the protocol.

Reviewers: ioeric

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

Differential Revision: https://reviews.llvm.org/D53398

llvm-svn: 344741