[clangd] Extract ClangdServer::Options struct.

Summary:
This subsumes most of the params to ClangdServer and ClangdLSPServer.
Adjacent changes:
 - tests use a consistent set of options, except when testing specific options
 - tests that previously used synchronous mode for convenience no longer do
 - added a runAddDocument helper to SyncAPIs to mitigate the extra code
 - rearranged main a bit to follow the structure of the options

Reviewers: ilya-biryukov

Subscribers: klimek, jkorous-apple, ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D44088

llvm-svn: 326719