[clangd] Parse all comments in Sema and completion.

Summary:
And add tests for the comment extraction code.

clangd will now show non-doxygen comments in completion for results
coming from Sema and Dynamic index.
Static index does not include the comments yet, I will enable it in
a separate commit after investigating which implications it has for
the size of the index.

Reviewers: sammccall, hokein, ioeric

Reviewed By: sammccall

Subscribers: klimek, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D46002

llvm-svn: 332460