[Issue 8285] Error passing CTFE-generated string slice to template value parameter

[Max Samukha]

Another test case. The error is different but the root cause must 
be the same.

string bar()
{
     string s = "ab";
     return s[0..$];
}

template T1()
{
     enum T1 = bar()[0..$]; // error
}

string baz()
{
     return T1!();
}

string foo(string s)
{
     return s;
}

static assert(foo(baz()) == "ab");

void main()
{
}

Error: variable __dollar cannot be read at compile time