swift234

Add support to the AST for multiple @semantic @attributes.

This is not wired up to SIL yet so whichever is the first value will
take precedence. We already support multiple values at the SIL level, but at the
SIL level the last value takes precedence.

Per Doug's request I added an optional transform range templated on the
attribute. This will make it easy to get all attributes from the AST of a
specific kind.