[clangd] Index symbols share storage within a slab.

Summary:
Symbols are not self-contained - it's only safe to hand them out if you
guarantee the lifetime of the underlying data.

Before this lands, I'm going to measure the before/after memory usage of the
LLVM index loaded into memory in a single slab.

Reviewers: hokein

Subscribers: klimek, ilya-biryukov, cfe-commits

Differential Revision: https://reviews.llvm.org/D41483

llvm-svn: 321272