Rustc-dev-guide: Add "landing pads" to the glossary

[mark-i-m]

[camelid]

Is a "landing pad" a link to the section that discusses the term?

[igaray]

I was looking at this the other day, my interpretation of the issue is that it is just adding to the glossary a general definition of landing pad and contextualize it somewhat for the rustc.

I was thinking of something like "A section of user code intended to catch, or otherwise clean up after, an exception."

[jyn514]

https://llvm.org/docs/ExceptionHandling.html#overview