[clangd] Add static index for the global code completion.

Summary:
Use the YAML-format symbols (generated by the global-symbol-builder tool) to
do the global code completion.
It is **experimental** only , but it allows us to experience global code
completion on a relatively small project.

Tested with LLVM project.

Reviewers: sammccall, ioeric

Reviewed By: sammccall, ioeric

Subscribers: klimek, ilya-biryukov, cfe-commits

Differential Revision: https://reviews.llvm.org/D41668

llvm-svn: 322191