[clangd] Run SignatureHelp using an up-to-date preamble, waiting if needed.

Summary:
After code completion inserts a header, running signature help using the old
preamble will usually fail. So we add support for consistent preamble reads.

Reviewers: ilya-biryukov

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

Differential Revision: https://reviews.llvm.org/D51438

llvm-svn: 341076