[clangd] Add a new highlighting kind for typedefs

Summary:
We still attempt to highlight them as underlying types, but fallback to
the generic 'typedef' highlighting kind if the underlying type is too
complicated.

Reviewers: hokein

Reviewed By: hokein

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67290

llvm-svn: 371402