[AArch64] Break up AArch64FrameLowering::emitPrologue (NFCI) (#157485)

`emitPrologue` was almost 1k SLOC, with a large portion not actually
related to emitting the vast majority of prologues.

This patch creates a new class `AArch64PrologueEmitter` for emitting the
prologue, which keeps common state/target classes as members. This makes
adding methods that handle niche cases easy, and allows methods to be
marked "const" when they don't redefine flags/state.

With this change, the core "emitPrologue" is around 275 LOC, with cases
like Windows stack probes or Swift frame pointers split into routines.
This makes following the logic much easier.

Author: MacDue