[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