[clangd] Add a tool to build YAML-format global symbols.

Summary:
The tools is used to generate global symbols for clangd (global code completion),
The format is YAML, which is only for **experiment**.

Usage:
./bin/global-symbol-builder </path/to/llvm-dir> > global-symbols.yaml

TEST:
used the tool to generate global symbols for LLVM (~72MB).

Reviewers: sammccall

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D41491

llvm-svn: 321358