[LangRef] No target-specific size limit for atomics (#136864)

According to the current LangRef, atomics of sizes larger than a
target-dependent limit are non-conformant IR. Presumably, that size
limit is `TargetLoweringBase::getMaxAtomicSizeInBitsSupported()`. As a
consequence, one would not even know whether IR is valid without
instantiating the Target backend.

To get around this, Clang's CGAtomic uses a constant "16 bytes" for the
maximally supported atomic. The verifier only checks the power-of-two
requirement.

In a discussion with jyknight, the intention is rather that the
AtomicExpandPass will just lower everything larger than the
target-supported atomic sizes to libcall (such as `__atomic_load`).
According to this interpretation, the size limit needs only be known by
the lowering and does not affect the IR specification.

The original "target-specific size limit" had been added in
59b66883eacbc62a09c09f08bcbfdce7af46cf31. The LangRef change is needed
for #134455 because otherwise frontends need to pass a TargetLowering
object to the helper functions just to know what the target-specific
limit is.

This also changes the LangRef for atomicrmw. Are there libatomic
fallbacks for these? If not, LLVM-IR validity still depends on
instantiating the actual backend. There are also some intrinsics such as
`llvm.memcpy.element.unordered.atomic` that have this constraint but do
not change in this PR.