Based off the PlaygroundTransform, this new ASTWalker leaves calls to __builtin_pc_before and __builtin_pc_after before and after a user would expect a program counter to enter a range of source code.
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