Make sure that only expands existing response files (#409)

Otherwise the parameter is more than likely a prefix of an `@rpath` so
it is probably better off leaving as it is.

This simply ports the following fix:
bazelbuild/bazel#13148

Resolves #408

Author: ormandi

toolchain/osx_cc_wrapper.sh.tpl

@@ -86,7 +86,7 @@ function sanitize_option() {
 
 cmd=()
 for ((i = 0; i <= $#; i++)); do
-  if [[ ${!i} == @* ]]; then
+  if [[ ${!i} == @* && -r "${i:1}" ]]; then
     while IFS= read -r opt; do
       if [[ ${opt} == "-fuse-ld=ld64.lld" ]]; then
         cmd+=("-fuse-ld=lld")