[clangd] Split Preamble.h out of ClangdUnit.h. NFC

Summary:
Add comment describing use of preamble in clangd.
Remove deps on ClangdUnit.h where possible.

Subscribers: mgorny, ilya-biryukov, javed.absar, MaskRay, jkorous, mgrang, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67117

llvm-svn: 370843