[clangd] Handle InitializeParams and store rootUri

Summary:
The root Uri is the workspace location and will be useful in the context of
indexing. We could also add more things to InitializeParams in order to
configure Clangd for C/C++ sepecific extensions.

Reviewers: ilya-biryukov, bkramer, krasimir, Nebiroth

Reviewed By: ilya-biryukov

Subscribers: ilya-biryukov

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D38093

llvm-svn: 314309