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

[Bruno De Fraine via llvm-commits]

brunodf-snps wrote:

In this version, the passes (NewPM and legacy) create a new SROA object for each function, so the SROA data structures are reallocated in between functions. If the allocation has an impact on compile-time, the pass objects could keep an SROA member (the NewPM SROAPass with a pimpl pattern).

https://github.com/llvm/llvm-project/pull/72846