Fix ref-counting bug in EnumType copy ctor

Which was used by the new ShallowClone method and causes multiple enum
declarations/definitions to now crash.  Such code is not typically
seen in scripts, but being able to define an enum in both .bif and
.bro files has historically been supported.

testing/btest/Baseline/language.enum-multiple-decls/out

@@ -0,0 +1 @@
+ONE

testing/btest/language/enum-multiple-decls.zeek

@@ -0,0 +1,10 @@
+# @TEST-EXEC: zeek -b %INPUT >out
+# @TEST-EXEC: btest-diff out
+
+# At the moment, it's legal to allow a double definitions.  Internally, the
+# reason/comment is: "so that we can define an enum both in a *.bif and *.zeek
+# for avoiding cyclic dependencies."
+
+type myenum: enum { ONE = 0x01 };
+type myenum: enum { ONE = 0x01 };
+print ONE;