[clangd] Do not build AST if no diagnostics were requested

Summary:
It can be removed from the cache before the first access anyway, so
building it can be a waste of time.

Reviewers: ioeric

Reviewed By: ioeric

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

Differential Revision: https://reviews.llvm.org/D49991

llvm-svn: 338378