[AST Matchers] Update dump_ast_matchers.py to query only class index page.

It may be possible to avoid relying on accessing many individual class pages,
by instead scanning the class index page at
https://clang.llvm.org/doxygen/classes.html. This updates the script to do so,
and includes updates to `LibASTMatchersReference.html` generated by the
modified script.

Reviewed By: aaron.ballman, sammccall

Differential Revision: https://reviews.llvm.org/D111332