cc_wrapper.sh.tpl has comment referring to old approach that has long been replaced

I happened to be reading the `cc_wrapper.sh.tpl` file, which has the following comment at the top:

```
# OS X relpath is not really working. This is a wrapper script around gcc
# to simulate relpath behavior.
#
# This wrapper uses install_name_tool to replace all paths in the binary
# (bazel-out/.../path/to/original/library.so) by the paths relative to
# the binary. It parses the command line to behave as rpath is supposed
# to work.
#
# See https://blogs.oracle.com/dipol/entry/dynamic_libraries_rpath_and_mac
# on how to set those paths for Mach-O binaries.
```

This comment appears to be referring to the solution implemented in the initial version of `cc_wrapper.sh.tpl` in [this commit](https://github.com/bazel-contrib/toolchains_llvm/commit/408474baa30039b6c5ba35c9545e2edcf644bb3d#diff-d0bdf5b1cdb738b12a358c7a5bf3b7f70be7678bdc84cd21fbc09d7b42193419). That solution was completely replaced with a different approach in [this commit](https://github.com/bazel-contrib/toolchains_llvm/commit/e4b62c9a4f123cf25701ce0760fe050fc4e293b3#diff-d0bdf5b1cdb738b12a358c7a5bf3b7f70be7678bdc84cd21fbc09d7b42193419), however. The comment still refers to the old approach, which can be confusing when trying to understand what the wrapper script is doing.

Start in cc_wrapper.sh.tpl and compare its top comment with the current implementation and the two referenced commits. Update the comment so it describes the approach actually used by the wrapper; the work is done when no text refers to the replaced implementation and the file remains consistent with its behavior.

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