[clangd] Split clangd into library+executable (mainly for unit tests).

Summary:
This commit itself doesn't add any unit tests, but one that does will
follow shortly.

Reviewers: krasimir, bkramer

Reviewed By: bkramer

Subscribers: mgorny, klimek, cfe-commits

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D33395

llvm-svn: 303616