Provide an attribute, objc_method_family, to allow the inferred family

of an Objective-C method to be overridden on a case-by-case basis.  This
is a higher-level tool than ns_returns_retained &c.;  it lets users specify
that not only does a method have different retain/release semantics, but
that it semantically acts differently than one might assume from its name.
This in turn is quite useful to static analysis.

llvm-svn: 126839