This PR adds `CommandLine.executablePath`. We have use cases for this property
in Swift Testing, Swift Argument Parser, Foundation, and other places; three use
cases a stdlib API makes.
Need to consider how best to handle back-deployment on Apple platforms. It
should be possible to support back-deployment since the underlying crt_extern
function has been present for decades, however calling it may prove non-trivial.
I haven't made an attempt at it yet.
Pending Swift Evolution review. We can mark this `@_spi` or underscored in the
mean time.
Jonathan Grynspan
6c8d8767d5