[clangd] Truncate SymbolID to 16 bytes.

Summary:
The goal is 8 bytes, which has a nonzero risk of collisions with huge indexes.
This patch should shake out any issues with truncation at all, we can lower
further later.

Reviewers: ioeric

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

Differential Revision: https://reviews.llvm.org/D53587

llvm-svn: 345113