Formalizing @cdecl

[woolsweater (Josh Caswell)]

@jrose

-emit-c-header , even though it will sometimes have ObjC in it? -emit-header feels a bit too generic. -emit-interoperation-header is verbose without solving the generic. Suggestions?

Two flags?

-emit-c-header -- warns/errors if you have ObjC declarations
-emit-objc-header -- happy with any combination of @cdecl or @objc declarations