[libcxx] [www] Cleanup links to be https.
Change http://wg21.link to https://wg21.link .
Change http://cplusplus.github.io/LWG/lwg-defects.html# to https://wg21.link/lwg .
Semi-manually change http://www.open-std.org and http://isocpp.org papers to https://wg21.link .
Manually change http links to https.
Fix broken link for LLVM Bugzilla.
Strip trailing whitespace.
Change an absolute link to cxx1z_status.html to be relative.