[include-mapping] Fix parsing of html_book_20190607.zip...

[include-mapping] Fix parsing of html_book_20190607.zip (https://en.cppreference.com/w/File:html_book_20190607.zip). Skip entries that have been added to the index (C++20 symbols), but the corresponding pages for which have not been created yet.

Differential Revision: https://reviews.llvm.org/D141509