[Atomics] warn about atomic accesses using libcalls

If an atomic variable is misaligned (and that suspicion is why Clang emits
libcalls at all) the runtime support library will have to use a lock to safely
access it, with potentially very bad performance consequences. There's a very
good chance this is unintentional so it makes sense to issue a warning.

Also give it a named group so people can promote it to an error, or disable it
if they really don't care.

llvm-svn: 330566