[4.2] [PlaygroundTransform] Replace "$builtin" with "__builtin".

[cwakamo]

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 is the swift-4.2-branch equivalent of #14257. I'll add Swift 4.2 nomination information after it passes testing.

This addresses rdar://problem/36031860.

[cwakamo]

Please test with this PR:

apple/swift-lldb#751

@swift-ci Please test

[cwakamo]

@jrose-apple I would greatly appreciate it if you could re-review this for Swift 4.2 nomination purposes!

[jrose-apple]

It looks fine to me! Does still need the nomination process, though.

[cwakamo]

• Explanation: Currently, the playground transform uses symbols like $builtin_log_with_id and others as the functions the compiler inserts calls to when running the transform. This means that playgrounds must be compiled with $-identifiers enabled, but this is suboptimal and effectively means that playgrounds must be built in a debugger-like context. To support more easily building with the playground transform with an unmodified compiler, as well as to bring the playground transform in alignment with the related PC macro, this change switches the playground transform to using functions named with a builtin* prefix instead of a $builtin* prefix. This change also updates lldb to use builtin* when setting up the playground transform instead of $builtin.
• Scope of Issue: Affects the playground transform and lldb playground expressions.
• Origination: The original design of playgrounds used $builtin_ because it was only invoked from the debugger.
• Risk: Low
• Reviewed By: Swift changes reviewed by @jrose-apple, and LLDB changes reviewed by @dcci and @jimingham
• Testing: Update the Swift and LLDB test suites to use _builtin* and observed no regressions.