[AST] Added a helper to extract a user-friendly text of a comment.

Summary:
The helper is used in clangd for documentation shown in code completion
and storing the docs in the symbols. See D45999.

This patch reuses the code of the Doxygen comment lexer, disabling the
bits that do command and html tag parsing.
The new helper works on all comments, including non-doxygen comments.
However, it does not understand or transform any doxygen directives,
i.e. cannot extract brief text, etc.

Reviewers: sammccall, hokein, ioeric

Reviewed By: ioeric

Subscribers: mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D46000

llvm-svn: 332458