Add some basic type checking for attributes ns_returns_retained and

cf_returns_retained. Currently this attribute can now be applied to any
Objective-C method or C function that returns a pointer or Objective-C object
type.

Modify the tablegen definition of diagnostic 'warn_attribute_wrong_decl_type' to
expect that the diagnostics infrastructure will add quotes around the attribute
name when appropriate. Alonq with this change, I modified the places where this
warning is issued to passed the attribute's IdentifierInfo* instead of having a
hard-coded C constant string.

llvm-svn: 71718