Parser: Parse operator decls.

At the top level, if 'operator' is followed by 'infix', 'prefix', or 'postfix', consider it a contextual keyword, and parse an operator decl following it that looks like:

  operator {infix|postfix|prefix} <+> {
    attributes…
  }

Prefix and postfix operator decls currently admit no attributes. Infix operators have 'associativity {left|right|none}' and 'precedence <int>' attributes.

This patch implements parsing for operator declarations but does not yet attach the declared attributes to func decls for the operators.

Swift SVN r4596

lib/IRGen/GenClass.cpp

@@ -731,6 +731,9 @@ void IRGenModule::emitClassDecl(ClassDecl *D) {
     case DeclKind::Protocol:
     case DeclKind::OneOfElement:
     case DeclKind::Extension:
+    case DeclKind::InfixOperator:
+    case DeclKind::PrefixOperator:
+    case DeclKind::PostfixOperator:
       llvm_unreachable("decl not allowed in class!");
 
     // We can have meaningful initializers for variables, but