[clangd] Collect and store expected types in the index

Summary:
And add a hidden option to control whether the types are collected.
For experiments, will be removed when expected types implementation
is stabilized.

The index size is almost unchanged, e.g. the YAML index for all clangd
sources increased from 53MB to 54MB.

Reviewers: ioeric, sammccall

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D52274

llvm-svn: 347560