[ast-printer] Don't emit 'infix' attribute for decls

[swiftix]

Textual SIL containing something like: infix static func ==(a: T, b: T) -> Bool cannot be parsed and results in an error like:

error: 'infix' modifier is not required or allowed on func declarations

Interestingly enough, prefix and postfix attributes do not result in the same kind of errors.

[swiftix]

@swift-ci please test

[swiftix]

@jrose-apple @DougGregor Please have a look at the fix. It is trivial.

But I wonder if it is intended that only infix cannot be parsed, but prefix and postfix are OK?

[DougGregor]

LGTM. prefix and postfix are needed because both are unary, so you need to distinguish between them.

[swift-ci]

Build failed
Jenkins build - Swift Test Linux Platform
Git Commit - 9a7d28a
Test requested by - @swiftix

[swiftix]

@swift-ci please smoke test Linux

[swiftix]

please test linux failure seems to be totally unrelated.