Currently, the playground transform requires the use of dollar-identifiers as the functions are prefixed with "$builtin".
This commit removes that requirement by replacing "$builtin" with "__builtin".
This aligns with the PC macro.
This addresses <rdar://problem/36031860>.
This converts the instances of the pattern for which we have a proper
substitution in lit. This will make it easier to replace it
appropriately with Windows equivalents.
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.
