[clangd] Shard preamble symbols in dynamic index

Summary:
This reduces memory usage by dynamic index from more than 400MB to 32MB
when all files in clang-tools-extra/clangd/*.cpp are active in clangd.

Reviewers: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77732