Clang: Use document visibility as jobrequest conditions

This is in preparation for a follow-up change that will add jobs that
also operate on invisible documents.

Change-Id: I2a0fc3b95cc5ab2e267429134a534df64c901376
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>
Reviewed-by: Marco Bubke <marco.bubke@qt.io>

src/tools/clangbackend/ipcsource/clangjobrequest.h

@@ -56,8 +56,10 @@ public:
     };
 
     enum class Condition {
-        NoCondition,
-        CurrentDocumentRevision,
+        NoCondition             = 1 << 0,
+        DocumentVisible         = 1 << 1,
+        DocumentNotVisible      = 1 << 2,
+        CurrentDocumentRevision = 1 << 3,
     };
     Q_DECLARE_FLAGS(Conditions, Condition)