Parse: Accept @available(DomainName).

For now `-enable-experimental-feature CustomAvailability` is required to accept
this spelling, though.

Author: tshortli

lib/Parse/ParseDecl.cpp

@@ -537,7 +537,8 @@ ParserResult<AvailableAttr> Parser::parseExtendedAvailabilitySpecList(
     }
   }
 
-  if (!AnyAnnotations) {
+  if (!AnyAnnotations &&
+      !Context.LangOpts.hasFeature(Feature::CustomAvailability)) {
     diagnose(Tok.getLoc(), diag::attr_expected_comma, AttrName,
              /*isDeclModifier*/ false);
   }

test/attr/attr_availability_custom_domains.swift

@@ -8,7 +8,6 @@
 
 // REQUIRES: swift_feature_CustomAvailability
 
-// expected-error@+1 {{expected ',' in 'available' attribute}}
 @available(EnabledDomain) // expected-warning {{unknown platform 'EnabledDomain' for attribute 'available'}}
 func availableInEnabledDomain() { }
 
@@ -18,11 +17,9 @@ func availableInDisabledDomain() { }
 @available(RedefinedDomain, deprecated, message: "Use something else") // expected-warning {{unknown platform 'RedefinedDomain' for attribute 'available'}}
 func availableInRedefinedDomain() { }
 
-// expected-error@+1 {{expected ',' in 'available' attribute}}
 @available(DynamicDomain) // expected-warning {{unknown platform 'DynamicDomain' for attribute 'available'}}
 func availableInDynamicDomain() { }
 
-// expected-error@+1 {{expected ',' in 'available' attribute}}
 @available(UnknownDomain) // expected-warning {{unknown platform 'UnknownDomain' for attribute 'available'}}
 func availableInUnknownDomain() { }