This patch introduces `--enable-experimental-noncopyable-generics` for
the build script. It replaces
`--swift-stdlib-experimental-noncopyable-generics`
The old build option only enables the feature when building the
stdlib, but if we've built the stdlib with NoncopyableGenerics, the
compiler should be hardwired to have that feature enabled, too.
This patch also introduces the `noncopyable_generics` lit parameter, so
that tests assuming the system was built with the feature can live
in-tree and be tested, if they specify `REQUIRES: noncopyable_generics`.