swift234

[build-script] Change the default SWIFT_SOURCE_ROOT and SWIFT_BUILD_ROOT.

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