Introduce some basic support for defining specific language features
that can be checked by name, e.g.,
#if $AsyncAwait
// use the feature
#endif
For backward compatibility with older compilers, to actually prevent
the parser from parsing, one will have to do a Swift compiler version
check, even though the version number doesn't matter. For example:
#if compiler(>=5.3) && $AsyncAwait
// use the feature
#endif
