swift234

[Decl Attributes] Change DECL_ATTR to specify serialization constraints for attributes.

Via preprocessor goop, Serialization.cpp generates a set of static
functions that can be used to verify if a set of attributes
can be serialized for a particular declaration.

This design forces the author of the attribute to specify up front
whether or not an attribute is supported on a given declaration
kind.  We can possibly hoist this into semantic analysis as well.

These N separate functions do conceptually replace a simple
variadic template implementation.  I'm fine with alternatives,
but the goal was to provide a way for the author of new 
attributes to describe the requirements in one place: Attr.def.

Swift SVN r15470