allowing zero-argument runtime-only struct constructors

[kenji hara]

Some days ago, I created patch, and posted pull request #72.
http://d.puremagic.com/issues/show_bug.cgi?id=6036
https://github.com/D-Programming-Language/dmd/pull/72

This patch fixes some opCall problem, and allow a zero-argument
default constructor on stack allocation, even exists multiple-argument
explicit constructors.

>        // Calls zero-argument constructor function,
>        // due to parentheses after type.
>        // Compile-time error if no zero-argument constructor defined.
>        MyStruct ms3 = MyStruct();
This will be passing compilation with my patch (unless zero-argument
static opCall is defined).

>        // Calls zero-argument constructor function,
>        // due to parentheses after type.
>        // Compile-time error if no zero-argument constructor defined.
>        // This behavior CANNOT BE GOTTEN with new and opCall in current
>        // versions of D to my knowledge.
>        MyStruct *ms4 = new MyStruct();
Unfortunately, my patch doesn't fix this behavior.
Please post a new report into bugzilla with test cases.

Kenji Hara