[clangd] DidChangeConfiguration Notification

Summary:

Implementation of DidChangeConfiguration notification handling in
clangd.  This currently only supports changing one setting: the path of
the compilation database to be used for the current project.   In other
words, it is no longer necessary to restart clangd with a different
command line argument in order to change the compilation database.

Reviewers: malaperle, krasimir, bkramer, ilya-biryukov

Subscribers: jkorous-apple, ioeric, simark, klimek, ilya-biryukov, arphaman, rwols, cfe-commits

Differential Revision: https://reviews.llvm.org/D39571



Signed-off-by: Simon Marchi <simon.marchi@ericsson.com>
Signed-off-by: William Enright <william.enright@polymtl.ca>
llvm-svn: 325784