[NFC][SROA] Remove implementation details from SROA header (#72846)

This moves the SROA implementation from SROAPass into a separate SROA
class that is defined in the cpp file, and reduces the SROAPass class to
a thin NewPM wrapper. This allows to remove all implementation details
from the SROA header, and the SROALegacyPass can wrap the SROA class
instead of the NewPM SROAPass.

The trigger for this change is a GCC warning about visibility of
implementation details in the SROA header after D138238. Credits to
Nikita Popov for suggesting this reorganization.