[clangd] Replace StringRef in SymbolLocation with a char pointer.

Summary:
This would save us 8 bytes per ref, and buy us ~40MB in total
for llvm index (from ~300MB to ~260 MB).

The char pointer must be null-terminated, and llvm::StringSaver
guarantees it.

Reviewers: sammccall

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

Differential Revision: https://reviews.llvm.org/D53427

llvm-svn: 346852