[Issue 24007] New: function/delegate literals cannot specify linkage

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24007

          Issue ID: 24007
           Summary: function/delegate literals cannot specify linkage
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: qs.il.paperinik at gmail.com

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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