[clangd] Add test for changing build configuration

Summary:
This patch adds tests for the two ways of changing build configuration
(pointing to a particular compile_commands.json):

- Through the workspace/didChangeConfiguration notification.
- Through the initialize request.

Subscribers: ilya-biryukov, ioeric, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D50255

llvm-svn: 338914