[Issue 24789] New: Disallow body-less, non-extern, local functions

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24789

          Issue ID: 24789
           Summary: Disallow body-less, non-extern, local functions
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: minor
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: b2.temp at gmx.com

Given this small program, which compiles and run without error:

```d
module m;

void oops(){}

void main() {
    void oops();
}  
```

there could be an error for `main.oops` because 

1. local body-less funcdecls have no use.
2. possibly the author was super-tired and meant to call `m.oops`.

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