Don't make class SementicContext::Empty final.

The class does not implement the pure virtual methods so can
not be considered 'final'. Clang 10 and 11 complain about this.

Context: chipsalliance/Surelog#3081
Signed-off-by: Henner Zeller <h.zeller@acm.org>

runtime/Cpp/runtime/src/atn/SemanticContext.h

@@ -92,7 +92,7 @@ namespace atn {
     return !operator==(lhs, rhs);
   }
 
-  class ANTLR4CPP_PUBLIC SemanticContext::Empty final : public SemanticContext{
+  class ANTLR4CPP_PUBLIC SemanticContext::Empty : public SemanticContext{
   public:
     /**
      * The default {@link SemanticContext}, which is semantically equivalent to