[clangd] Initial implementation of expected types

Summary:
Provides facilities to model the C++ conversion rules without the AST.
The introduced representation can be stored in the index and used to
implement type-based ranking improvements for index-based completions.

Reviewers: sammccall, ioeric

Reviewed By: sammccall

Subscribers: malaperle, mgorny, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D52273

llvm-svn: 347559