Much like attributes values, keywords are known beforehand and never
change. Instead of relying on handcrafted string we could centralize
everything in one place. We may require to check whether a word is a
keyword, which can now be done easily thanks to the keyword set.
gcc/rust/ChangeLog:
* Make-lang.in: Add rust-keyword-values.cc to the list.
* util/rust-keyword-values.cc: New file.
* util/rust-keyword-values.h: New file.