[clangd] Use ToolExecutor to write the global-symbol-builder tool.

Summary:
This enables more execution modes like standalone and Mapreduce-style execution.

See also D41729

Reviewers: hokein, sammccall

Subscribers: klimek, ilya-biryukov, cfe-commits

Differential Revision: https://reviews.llvm.org/D41730

llvm-svn: 322084