Add mapping from removed section labels to new location of content in xrefs

[zygoloid]

In the xrefs section, we should list all the C++14 section labels that no longer exist in C++17, along with an indication of where the content is now.

[tkoeppe]

Sounds good. We could start by extracting the labels now and from the C++14 FDIS to get a list of candidates.

[zygoloid]

After we branch, I'd like for us to maintain a "differences from C++17" list in xrefs incrementally. Might also be good to have a "differences from post-C++17 drafts" section that we remove for C++20 DIS.