[llvm][TableGen] Fix value description made by OptRSTEmitter

When this was ported to clang-tblen for https://reviews.llvm.org/D123682,
some of the refactoring for the clang copy was backported to llvm,
but used .front instead of .back as clang does.

This means that if you have values "a, b, c" you get
"must be 'a', ' b' or 'a'." instead of "must be 'a', ' b' or 'c'.".