bug#63918: 28.2; re-builder rx fails to match

[Mattias Engdegård]

The problem is that re-builder's handling of rx is badly designed, and you are 
not the first to be annoyed by this. In 'rx' mode, re-builder expects you to 
write a Lisp expression that can be used as argument to `rx-to-string`, 
something that nobody wants to do. In your case, instead of

  (rx (or "a" "b") "c")

you would have to write something like

  '(seq (or "a" "b") "c")

Note the quote (because the expression is evaluated) and `seq` (because 
`rx-to-string` only takes a single rx expression as input; `rx` forms an 
implicit `seq` of its arguments).

Previous bugs: 

https://debbugs.gnu.org/cgi/bugreport.cgi?bug=60196
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=54107