Although actor initializers can be classified as convenience initializers by
the internals of the compiler, they are not written that way in source and
should not be printed with the `convenience` keyword.
Resolves rdar://130926278.
The `convenience` keyword is not accepted on actor initializers because the
compiler infers this status automatically and actors cannot be subclassed.
However, internally the compiler still computes whether an actor init is a
convenience init and this implicit status has been leaking through accidentally
in printed `.swiftinterface` files. This was noticed because in Swift 6 the
presence of the keyword is diagnosed as an error, making `.swiftinterface`
files unparseable for modules containing actors with convenience inits.
For Swift 6, I'm just going to suppress the error in `.swiftinterface` files
regardless of language mode. In future releases of the compiler, though, it can
stop printing the `convenience` keyword on these inits altogether, though.
Resolves rdar://130857256.
