grammar allows missing function body for AutoFuncDeclaration

[dlangBugzillaToGithub]

Dennis (@dkorpel) reported this on 2020-01-14T21:08:37Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=20504

Description

This does not compile:
```
void foo()() {
    nothrow bar() {} // allowed
    nothrow bar(); // should parse according to spec
}
```
The spec says that in an AutoFuncDeclaration a MissingFunctionBody is allowed, yet even before instantiating template foo it raises an error:
Error: function declaration without return type.

I suggest only allowing SpecifiedFunctionBody in an AutoFuncDeclaration since it would always be a semantic error anyways (you can't infer the return type without a function body).

https://dlang.org/spec/grammar.html#AutoFuncDeclaration
https://dlang.org/spec/grammar.html#FunctionBody