This default would never actually be used, because if you did check out your
Swift sources there it would look like a normal Swift checkout and we would
infer SWIFT_SOURCE_ROOT from that.
Also, clean up some comments based on suggestions from Dave.
Swift SVN r24606
SWIFT_SOURCE_ROOT is now auto-detected from the location of the build script
files. It looks for the following layouts:
$SWIFT_SOURCE_ROOT/<LLVM_CHECKOUT>/tools/<SWIFT_CHECKOUT>/utils/build-script
$SWIFT_SOURCE_ROOT/<SWIFT_CHECKOUT>/utils/build-script
and defaults back to ~/src/s (as before) if neither of those matches.
SWIFT_BUILD_ROOT now defaults to $SWIFT_SOURCE_ROOT/build, rather than
~/build/swift.
Swift SVN r24194