osx_cc_wrapper: Only expand existing response files (bazelbuild#15090)

Closes bazelbuild#13044
Applies the changes suggested in bazelbuild#13044 (comment)

Not all arguments starting with `@` represent response files, e.g. `-install_name @rpath/...` or `-Xlinker -rpath -Xlinker @loader_path/...` do not refer to response files and attempting to read them as such will fail the build.

Users don't always have control over these arguments, meaning transforming them to `-Wl,-install_name,@rpath/...` or `-Wl,-rpath,@loader_path/...` is not always an option. E.g. as described in bazelbuild#13044 (comment) where these flags are emitted by the Haskell compiler GHC (see bazelbuild#13044 (comment) for their reasoning).

With this change `osx_cc_wrapper` will only interpret arguments starting with `@` as response files if the corresponding file exists and is readable. This is analogous to the behavior defined in `wrapped_clang.cc`. See bazelbuild#13044 (comment) for discussion.

Closes bazelbuild#13148.

PiperOrigin-RevId: 436207868
(cherry picked from commit efb2b80)

Co-authored-by: Andreas Herrmann <andreash87@gmx.ch>

tools/cpp/osx_cc_wrapper.sh

@@ -53,7 +53,7 @@ function parse_option() {
 
 # let parse the option list
 for i in "$@"; do
-    if [[ "$i" = @* ]]; then
+    if [[ "$i" = @* && -r "${i:1}" ]]; then
         while IFS= read -r opt
         do
             parse_option "$opt"

tools/cpp/osx_cc_wrapper.sh.tpl

@@ -52,7 +52,7 @@ function parse_option() {
 
 # let parse the option list
 for i in "$@"; do
-    if [[ "$i" = @* ]]; then
+    if [[ "$i" = @* && -r "${i:1}" ]]; then
         while IFS= read -r opt
         do
             parse_option "$opt"