[clangd] Allow to merge symbols on-the-fly in global-symbol-builder

Summary:
The new mode avoids serializing and deserializing YAML.
This results in better performance and less memory usage. Reduce phase
is now almost instant.

The default is to use the old mode going through YAML serialization to
allow migrating MapReduce clients that require the old mode to operate
properly. After we migrate the clients, we can switch the default to
the new mode.

Reviewers: hokein, ioeric, kbobyrev, sammccall

Reviewed By: ioeric

Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D51155

llvm-svn: 340600