can't deprecate default construction

[dlangBugzillaToGithub]

Martin Nowak (@MartinNowak) reported this on 2015-05-04T17:17:01Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=14545

Description

cat > enh.d << CODE
struct Foo
{
    deprecated("use foo() instead")
    this();
}
CODE

dmd -c enh

----
Error: constructor enh.Foo.this default constructor for structs only allowed with @disable and no body
----

There is no mean to deprecate default construction of a struct before finally disabling it.
We should enhance the compiler so that the above declaration is valid and using `Foo foo;` would emit a deprecation warning.