[clangd] Ensure preamble outlives the AST

Summary:
In-memory preambles will not be copied anymore, so we need to make
sure they outlive the AST.

Reviewers: bkramer, sammccall, klimek

Reviewed By: sammccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D40301

llvm-svn: 318944