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

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.

Author: swiftix

lib/AST/Attr.cpp

@@ -322,6 +322,7 @@ bool DeclAttribute::printImpl(ASTPrinter &Printer, const PrintOptions &Options,
   case DAK_SynthesizedProtocol:
   case DAK_ShowInInterface:
   case DAK_Rethrows:
+  case DAK_Infix:
     return false;
   default:
     break;

test/IDE/print_ast_tc_decls.swift

@@ -585,7 +585,7 @@ struct d0200_EscapedIdentifiers {
   }
 // PASS_COMMON-NEXT: {{^}}  enum `enum` {{{$}}
 // PASS_COMMON-NEXT: {{^}}    case `case`{{$}}
-// PASS_COMMON-NEXT: {{^}}    infix static func ==(a: d0200_EscapedIdentifiers.`enum`, b: d0200_EscapedIdentifiers.`enum`) -> Bool
+// PASS_COMMON-NEXT: {{^}}    static func ==(a: d0200_EscapedIdentifiers.`enum`, b: d0200_EscapedIdentifiers.`enum`) -> Bool
 // PASS_COMMON-NEXT: {{^}}    var hashValue: Int { get }{{$}}
 // PASS_COMMON-NEXT: {{^}}  }{{$}}
 
@@ -1007,7 +1007,7 @@ enum d2000_EnumDecl1 {
 // PASS_COMMON: {{^}}enum d2000_EnumDecl1 {{{$}}
 // PASS_COMMON-NEXT: {{^}}  case ED1_First{{$}}
 // PASS_COMMON-NEXT: {{^}}  case ED1_Second{{$}}
-// PASS_COMMON-NEXT: {{^}}  infix static func ==(a: d2000_EnumDecl1, b: d2000_EnumDecl1) -> Bool
+// PASS_COMMON-NEXT: {{^}}  static func ==(a: d2000_EnumDecl1, b: d2000_EnumDecl1) -> Bool
 // PASS_COMMON-NEXT: {{^}}  var hashValue: Int { get }{{$}}
 // PASS_COMMON-NEXT: {{^}}}{{$}}