[clangd] Construct SymbolSlab from YAML format.

Summary: This will be used together with D40548 for the global index source (experimental).

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: klimek, mgorny, ilya-biryukov, cfe-commits, ioeric

Differential Revision: https://reviews.llvm.org/D41178

llvm-svn: 320694