[HLSL] Add test for export function redeclaration (#97370)

Related to #92812

Author: hekota

clang/test/SemaHLSL/export.hlsl

@@ -35,6 +35,12 @@ static void f9(); // expected-error {{static declaration of 'f9' follows non-sta
 static void f10(); // expected-note {{previous declaration is here}}
 export void f10(); // expected-error {{cannot export redeclaration 'f10' here since the previous declaration has internal linkage}}
 
+export void f11();
+void f11() {}
+
+void f12();           //   expected-note{{previous declaration is here}}
+export void f12() {}  //   expected-error{{cannot export redeclaration 'f12' here since the previous declaration is not exported}}
+
 export float V1; // expected-error {{export declaration can only be used on functions}}
 
 static export float V2; // expected-error{{expected unqualified-id}}