Xcode uses a different naming scheme for directories within
the xcodebuild directory. But it is safe to just delete
everything withing xcodebuild or out. Keep the soft clobber
for windows' build directory only, where subdirectories
follow the *release* and *debug* naming scheme.

BUG=chromium:403263
LOG=n
TBR=jochen@chromium.org

Review URL: https://codereview.chromium.org/955953002

Cr-Commit-Position: refs/heads/master@{#26852}