[clangd] Make background index less chatty

Summary:
It is producing too much input in non-verbose mode,
i.e. a message per indexed file

Reviewers: sammccall, kadircet

Reviewed By: sammccall

Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D56915

llvm-svn: 351563