Support Playgrounds in any Swift file

[jerrymarino]

It should be possible to compile and execute a playground for every Swift file.

A few important use cases come to mind that can leverage this:

• Simple programs which are executable: a program where logic is in a main.swift.

• Unit testing. Playgrounds are super applicable to unit testing, since it allows the user to see program state within a test case. This is easier to work with than the existing approach people have adpoted: copying all of the code they want to play with into a playground file.

I think this is a good addition and would make Swift development more interactive overall.

Caveats

Many Swift files don't contain don't contain code that executes, so for a large subset of files, this isn't 100% useful. For example, MyModel.swift is just a class definition.

Alternatives Considered

Copy all of the code in question into a playground.

Implementation

At the compiler level, we are just performing a transform that inserts the playground logging code in a module. I think this pass should work for any file. In my initial testing, it seems to work fine on some sample code.

Pass 1 will use compile_commands.json to get a base a compiler invocation and augment it to include the playground bits.