#184545 default-enables the IO sandbox in assert-builds. This causes
Clang using Polly to crash (#188568).
The issue is that `PassBuilder` uses `vfs::getRealFileSystem()` by
default which is considered a IO sandbox violation in the Clang process.
With this PR store the VFS from the `PassBuilder` from the original
`registerPollyPasses` call for creating other `PassBuilder` instances.
This PR also adds infrastructure for running Polly in `clang` (in
addition in `opt`). `opt` does not enable the sandbox such that we need
separate tests using Clang.
Closes: #188568
458f1aae8d