[www] Change URLs to HTTPS.

This changes most URLs in llvm's html files to HTTPS. Most changes were
search-and-replace with manual verification; some changes were manual.
For a few URLs, the websites were performing redirects or had changed
their anchors; I fixed those up manually. This consistently uses the
official https://wg21.link redirector. This also strips trailing
whitespace and fixes a couple of typos.

Fixes D69363.

There are a very small number of dead links for which I don't know any
replacements (they are equally dead as HTTP or HTTPS):

https://llvm.org/cmds/llvm2cpp.html
https://llvm.org/devmtg/2010-11/videos/Grosser_Polly-desktop.mp4
https://llvm.org/devmtg/2010-11/videos/Grosser_Polly-mobile.mp4
https://llvm.org/devmtg/2011-11/videos/Grosser_PollyOptimizations-desktop.mov
https://llvm.org/devmtg/2011-11/videos/Grosser_PollyOptimizations-mobile.mp4
https://llvm.org/perf/db_default/v4/nts/22463
https://polly.llvm.org/documentation/memaccess.html