Add test for enum with two autoload annotations

This is a heavily reduced case that shows the problem originally
reported in cms-sw/cmssw#42234.

cling/dict/enum/CMakeLists.txt

@@ -0,0 +1,8 @@
+# List only header.h which transitively includes enum.h - this leads to two
+# __attribute__((annotate("$clingAutoload$...")))
+ROOTTEST_GENERATE_DICTIONARY(enumDict header.h LINKDEF LinkDef.h NO_CXXMODULE)
+
+ROOTTEST_ADD_TEST(exec
+    MACRO execEnumDict.C
+    DEPENDS ${GENERATE_DICTIONARY_TEST}
+)

cling/dict/enum/LinkDef.h

@@ -0,0 +1 @@
+#pragma link C++ class std::vector<Enum>+;

cling/dict/enum/enum.h

@@ -0,0 +1,9 @@
+#ifndef ENUM_H
+#define ENUM_H
+
+enum Enum {
+  A = 1,
+  B = 2,
+};
+
+#endif

cling/dict/enum/execEnumDict.C

@@ -0,0 +1,5 @@
+#include "enum.h"
+
+int execEnumDict() {
+  return gSystem->Load("enumDict");
+}

cling/dict/enum/header.h

@@ -0,0 +1 @@
+#include "enum.h"