build-llvm: Fix only checking out the source

This is used if calling build-compiler-rt.sh or build-libcxx.sh with
the environment variable SYNC set.

build-llvm.sh

@@ -19,6 +19,7 @@ while [ $# -gt 0 ]; do
     fi
     shift
 done
+if [ -z "$CHECKOUT_ONLY" ]; then
     if [ -z "$PREFIX" ]; then
         echo $0 [--enable-asserts] [--full-llvm] dest
         exit 1
@@ -26,6 +27,7 @@ fi
 
     mkdir -p "$PREFIX"
     PREFIX="$(cd "$PREFIX" && pwd)"
+fi
 
 : ${CORES:=$(nproc 2>/dev/null)}
 : ${CORES:=$(sysctl -n hw.ncpu 2>/dev/null)}