Fix 23386 - Segfault on enum member UDA inside template

[dkorpel]

EnumMember.syntaxCopy calls UserAttributeDeclaration.syntaxCopy which will copy the AttribDeclaration.decl array, including the original EnumMember, resulting in endless recursion.

Parameter.syntaxCopy doesn't have this problem because the parser doesn't add the Parameter to the uda's decl list, and before #11527 neither did the parser add an EnumMember to the uda's decl. This PR restores the old behavior since it seems to be unrelated to the fix, though it's still not ideal that syntaxCopy doesn't protect against cycles in the AST.

Targeting master assuming stable is still closed (@ibuclaw)

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	23386	regression	Segfault on enum member UDA inside template

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