There were 2 things with the previous implementation that needed to be
improved here:

1. Apple Clang has a bug where it doesn't pass module compiler flags to
the underlying -cc1 invocation, so we have to manually pass them
directly to that invocation with -Xclang
2. The previous search script was too aggressive and slow for macOS. The
macOS SDK has tons of files that aren't headers, and tons of symlinks
pointing to other files within the SDK. This adds a fork in the script
to run a version that works with Apple SDKs. The time difference on my
machine is 41s->6s. 6s is still pretty long so if desired we can put
this behavior behind an env var for users to opt in with.

I've added a hermetic version of this to the apple_support toolchain,
but similar to the Linux setup here the modulemap file includes absolute
paths.

Closes #22259.

This reverts commit 1f1b4fd3.

Closes #22475.

PiperOrigin-RevId: 637969838
Change-Id: I7...