This can be used to opt out of TBAA for particular pointers without
breaking out the big hammer of asm volatile("" : : "r"(ptr) : "memory").
The underlying motivation is to allow the implementation of C++23's
We name this a TBAA-fence instead of something more generic to make it
clear that other alias analyses are allowed to see through the fence.
Additionally, the intrinsic returns a "safe-to-use" pointer rather than
a semantic guarantee of being a completely opaque ArgMemOnly intrinsic.
This enables some optimization opportunities: it lets us track uses more
precisely, so we could (e.g.) strip TBAA metadata from associated loads
and stores to enable store-to-load forwarding where beneficial. More
immediately, we don't need the stronger guarantees for
start_lifetime_as, so there's no reason to provide it out of the gate.