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

`emitPrologue` was almost 1k SLOC, with a large portion of that 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 off into their
own routines.

Author: MacDue