Allow the use of declarations whose names start with $ in all
modes. However, normal code cannot define new entities with names that
start with $: only the implementation can do that, e.g., for property
delegates.
Most tests were using %swift or similar substitutions, which did not
include the target triple and SDK. The driver was defaulting to the
host OS. Thus, we could not run the tests when the standard library was
not built for OS X.
Swift SVN r24504
For now, also check that -playground is off; once <rdar://problem/18090611>
is in we can drop this check and treat them as mostly orthogonal options.
Part of <rdar://problem/17994094>
Swift SVN r21385
