[RFC][clangd] Move preamble index out of document open critical path

We would like to move the preamble index out of the critical path.
This patch is an RFC to get feedback on the correct implementation and potential pitfalls to keep into consideration.

I am not entirely sure if the lazy AST initialisation would create using Preamble AST in parallel. I tried with tsan enabled clangd but it seems to work OK (at least for the cases I tried)

Reviewed By: kadircet

Differential Revision: https://reviews.llvm.org/D148088