Support nogc_construct of struct with disabled postblit.

[p0nce]

This one is a bit tricky.

The issue it fix is "not being able to nogc_construct a disabled-postblit struct on DMDFE >= 2111" (eg: LDC 1.41.0)

Here the placement new will act directly on the referenced object (instead of relying on declaring a temp initialized T and assign).

HOWEVER

Calling emplace on uninitialized memory is considered UB by numem, but it does visibly happen in numem and the code I replaced was doing a .init initialization with T tmp;.
So out of desire to not break anything, initializeAt(dst); was added.

I checked in an uncommited test that placement new does NOT initialize memory.

I think "nogc_construct" contract should read:

• calling nogc_construct on uninitialized object is UB" (though for now it works)
• calling nogc_construct on created object is illegal
  (EDIT: tests pass locally but yeah probably trickier than that)

[p0nce]

Can repro CI failure with DMD 2.111

[p0nce]

Sorry to ask, a git tag would one thing Dplug needs to depend on numem, since today all objects numem does all object creation/destruction in ~main. :)

[LunaTheFoxgirl]

Sure, will create a new tag in a bit.