Add -Wcustom-atomic-properties which warns if an atomic-by-default property...

Add -Wcustom-atomic-properties which warns if an atomic-by-default property has custom getter or setter.

The rationale is that it is highly likely that the user's getter/setter isn't atomically implemented. Off by default.
Addresses rdar://8782645.

-Wcustom-atomic-properties and -Wimplicit-atomic-properties are under the -Watomic-properties group.

llvm-svn: 124609