api-digester: Make unknown decl attrs a warning

Since the API digester is designed to tolerate them anyway.

Author: beccadax

include/swift/AST/DiagnosticsCommon.def

@@ -127,8 +127,8 @@ ERROR(sdk_node_unrecognized_node_kind,none,
       "unrecognized SDK node kind '%0'", (StringRef))
 ERROR(sdk_node_unrecognized_type_attr_kind,none,
       "unrecognized type attribute '%0' in SDK node", (StringRef))
-ERROR(sdk_node_unrecognized_decl_attr_kind,none,
-      "unrecognized declaration attribute '%0' in SDK node", (StringRef))
+WARNING(sdk_node_unrecognized_decl_attr_kind,none,
+        "unrecognized declaration attribute '%0' in SDK node", (StringRef))
 ERROR(sdk_node_unrecognized_decl_kind,none,
       "unrecognized declaration kind '%0' in SDK node", (StringRef))
 ERROR(sdk_node_unrecognized_accessor_kind,none,

test/api-digester/diagnostics.swift

@@ -13,17 +13,17 @@
 // CHECK: diagnostics.json:8:15: error: unrecognized SDK node kind 'Zyzyx'
 // CHECK: diagnostics.json:9:41: error: unrecognized type attribute 'fnord' in SDK node
 // CHECK: diagnostics.json:9:59: error: unrecognized type attribute 'Available' in SDK node
-// CHECK: diagnostics.json:10:39: error: unrecognized declaration attribute 'Fnord' in SDK node
-// CHECK: diagnostics.json:10:56: error: unrecognized declaration attribute 'inout' in SDK node
+// CHECK: diagnostics.json:10:39: warning: unrecognized declaration attribute 'Fnord' in SDK node
+// CHECK: diagnostics.json:10:56: warning: unrecognized declaration attribute 'inout' in SDK node
 // CHECK: diagnostics.json:11:19: error: unrecognized declaration kind 'Subroutine' in SDK node
 
 // Make sure we don't try to output a result:
 // NEGATIVE-NOT: "kind": "Root",
 
 // Older versions of the compiler used 'BackDeploy' for @_backDeploy. We now use
 // 'BackDeployed', but we should still support the old name.
-// NEGATIVE-NOT: error: unrecognized declaration attribute 'BackDeploy' in SDK node
+// NEGATIVE-NOT: {{warning|error}}: unrecognized declaration attribute 'BackDeploy' in SDK node
 // NEGATIVE-NOT: is now without @backDeployed
 
 // Should not have any errors in the diagnostics-compare file.
-// NEGATIVE-NOT: diagnostics-compare.json:{{.*}}: error:
+// NEGATIVE-NOT: diagnostics-compare.json:{{.*}}: {{warning|error}}: