[clangd] Receive compilationDatabasePath in 'initialize' request

Summary:
That way, as soon as the "initialize" is received by the server, it can start
parsing/indexing with a valid compilation database and not have to wait for a
an initial 'didChangeConfiguration' that might or might not happen.
Then, when the user changes configuration, a didChangeConfiguration can be sent.

Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>

Reviewers: malaperle

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

Differential Revision: https://reviews.llvm.org/D49833

llvm-svn: 338518