[clangd] Move clangd tests to clangd directory. check-clangd is no longer part of check-clang-tools.

Summary:
Motivation:
 - this layout is a pain to work with
 - without a common root, it's painful to express things like "disable clangd" (D61122)
 - CMake/lit configs are a maintenance hazard, and the more the one-off hacks
   for various tools are entangled, the more we see apathy and non-ownership.

This attempts to use the bare-minimum configuration needed (while still
supporting the difficult cases: windows, standalone clang build, dynamic libs).
In particular the lit.cfg.py and lit.site.cfg.py.in are merged into lit.cfg.in.
The logic in these files is now minimal.

(Much of clang-tools-extra's lit configs can probably be cleaned up by reusing
lit.llvm.llvm_config.use_clang(), and every llvm project does its own version of
LDPATH mangling. I haven't attempted to fix any of those).

Docs are still in clang-tools-extra/docs, I don't have any plans to touch those.

Reviewers: gribozavr

Subscribers: mgorny, javed.absar, MaskRay, jkorous, arphaman, kadircet, jfb, cfe-commits, ilya-biryukov, thakis

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61187

llvm-svn: 359424