[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

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@353054 91177308-0d34-0410-b5e6-96231b3b80d8