mixin template should be able to inject code as well as symbols

[dlangBugzillaToGithub]

Nicholas Wilson (@thewilsonator) reported this on 2018-12-22T09:46:27Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=19506

Description

Works
---
mixin template foo(bool b)
{
    int _impl() { writeln(b); return int.init; }
    int _ipml2 = _impl();
}
void main()
{
  mixin foo;
}
---
doesn't
---
mixin template foo(bool b)
{
        writeln(b);
}

void main()
{
  mixin foo;
}
---

see https://forum.dlang.org/post/txcblywfkrhpxnsbfydk@forum.dlang.org for the discovery of this idiom