[clangd] Do not report comments that only have special chars.

Summary:
Like the following:
  // -------
  // =======
  // *******

It does not cover all the cases, but those are definitely not very
useful.

Reviewers: sammccall, ioeric, hokein

Reviewed By: sammccall

Subscribers: MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D48171

llvm-svn: 334807