Otherwise, we can't successfully dry-run a toolchain on darwin.
I added an option enable-extract-symbol-dry-run-test that can be set to restore
the previous behavior and updating the BuildSystem unit tests that depend on
that behavior to pass that flag.
It makes sense to just set this here since we aren't testing that functionality
and enables us to test this if we aren't using one of the specified Xcodes
(which can be useful).
...before adding the logic to filter paths
In particular:
* print the list of files that `cpio` copies, so we can test explicitly
which files end up in the symroot (and also see those when asking for
a toolchain in PR testing)
* use `find` instead of `grep` to filter files we want symbols generated
for -- this is to avoid the script failing when there are no symbol to
process, especially in lit tests
* remove an unnecessary check for `swift-api-digester` -- this is now a
symlink to `swift-frontend` and we only process regular files.
Supports rdar://76865276
