[clangd] Truncate SymbolID to 8 bytes.

Summary:
This is our goal. It has a non-zero rick, but so far we haven't see any
collision (externally and internally).

Reviewers: sammccall

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

Differential Revision: https://reviews.llvm.org/D54622

llvm-svn: 347044