[clangd] Make in-memory CDB always available as an overlay, refactor.

Summary:
The new implementation is a GlobalCompilationDatabase that overlays a base.
Normally this is the directory-based CDB.
To preserve the behavior of compile_args_from=LSP, the base may be null.

The OverlayCDB is always present, and so the extensions to populate it
are always supported.

It also allows overriding the flags of the fallback command. This is
just unit-tested for now, but the plan is to expose this as an extension
on the initialize message. This addresses use cases like
https://github.com/thomasjo/atom-ide-cpp/issues/16

Reviewers: ilya-biryukov

Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D53687

llvm-svn: 345970