[Testing] Move clangd::Annotations to llvm testing support

Summary:
Annotations allow writing nice-looking unit test code when one needs
access to locations from the source code, e.g. running code completion
at particular offsets in a file. See comments in Annotations.cpp for
more details on the API.

Also got rid of a duplicate annotations parsing code in clang's code
complete tests.

Reviewers: gribozavr, sammccall

Reviewed By: gribozavr

Subscribers: mgorny, hiraditya, ioeric, MaskRay, jkorous, arphaman, kadircet, jdoerfert, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D59814

llvm-svn: 359179