Bug 24007 – function/delegate literals cannot specify linkage

There is no way to express the linkage for a [function literal](https://dlang.org/spec/expression.html#function_literals). I suggest to add [LinkageAttribute](https://dlang.org/spec/attribute.html#linkage) in the first two production rules for `FunctionLiteral`: ``` FunctionLiteral: function LinkageAttribute? RefOrAutoRef? Type? ParameterWithAttributes? FunctionLiteralBody2 delegate LinkageAttribute? RefOrAutoRef? Type? ParameterWithMemberAttributes? FunctionLiteralBody2 ``` (Replaced opt with `?` for better readability.) Linkage is niche enough to justify why it’s not needed in other production rules.

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