[clangd] Enable include insertion for static index

Summary:
This enables include insertion by adding canonical includes into
preambledata.

Reviewers: ioeric, ilya-biryukov

Subscribers: javed.absar, MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D57508

llvm-svn: 353054