[PCH] Add an option to not write comments into PCH

Summary:
Will be used in clangd, see the follow-up change.
Clangd does not use comments read from PCH to avoid crashes due to
changed contents of the file. However, reading them considerably slows
down code completion on files with large preambles.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D48942

llvm-svn: 336539