[clangd] Do not write comments into Preamble PCH

Summary:
To avoid wasting time deserializing them on code completion and
further reparses.

We do not use the comments anyway, because we cannot rely on the file
contents staying the same for reparses that reuse the prebuilt
preamble PCH.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: ioeric, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D48943

llvm-svn: 336540