[BoundsSafety] Provide unsafe memcpy/memset for migration

[mxms0]

Proposal

Add __unsafe_memcpy/__unsafe_memset (and possibly others) to ptrcheck.h a la:

uint8_t* __unsafe_indexable __unsafe_memcpy(uint8_t* __unsafe_indexable dst, uint8_t* __unsafe_indexable src, size_t size) { 
  memcpy(__unsafe_forge_bidi_indexable(uint8_t*, dst, size),
         __unsafe_forge_bidi_indexable(uint8_t*, src, size),
         size);
}

Could also be a macro, I suppose :)

The general pattern I've been going with as part of adoption is to get everything building with -fbounds-safety by marking nearly everything ABI-changing as __unsafe_indexable. This makes it easier to make localized improvements while still being able to build and run tests.

They'd still be clearly marked as unsafe, and so they'd be easily searchable/identifiable.

This also makes #pragma clang abi_ptr_attr set(unsafe_indexable) more attainable, and easier to adopt on per-line, per-function, per-TU granularities.

This request is mostly an adoption/ergonomics proposal, and as mentioned in discord, a temporary workaround exists to put these unsafe functions in a local header for adoption.