...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
