Xcode 6.4 and Xcode 7 beta 4 both stopped working with OS X El Cap beta 6. This seems to fix it. See comments for deets. Requires a sudo password, don't be freaked out.

save_xcode_for_el_cap.bash

#!/usr/bin/env bash
#
# Based on original solution by user lembacon, here:
# https://forums.developer.apple.com/thread/13170?start=105&tstart=0
# Watch fix added by user Richie__.
#
# Created by Clay Bridges, 5 Aug 2015

move_dyld_sim()
{
    if [ -f dyld_sim ];
    then
      sudo mv dyld_sim dyld_sim.orig
    else
      echo "ERROR: dyld_sim does not exist"
      exit 256
    fi
}

APP=$1

if [ -e "$APP" ];
then
	echo "Fixing Xcode instance \"$APP\" for El Capitan beta 6"
else
	echo "ERROR: first argument \"$APP\" must be a path to an Xcode app"
	echo "Usage: $0 <path-to-xcode>"
	exit 256
fi

PLATFORMS=$APP/Contents/Developer/Platforms
IOS_LIB_DIR=$PLATFORMS/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk/usr/lib
WATCH_LIB_DIR=$PLATFORMS/WatchSimulator.platform/Developer/SDKs/WatchSimulator.sdk/usr/lib

if [ -d "$IOS_LIB_DIR" ];
then
	cd $IOS_LIB_DIR
    move_dyld_sim
    echo "Fixed iPhoneSimulator"
else
    printf "ERROR: could not find iOS Simulator directory:\n   $IOS_LIB_DIR\n"
    exit 256
fi

# I think this only applies to Xcode 7
if [ -d "$WATCH_LIB_DIR" ];
then
    cd $WATCH_LIB_DIR
    move_dyld_sim
    echo "Fixed WatchSimulator"
fi