Ensure that the executable module is ModuleList[0] (#78360)

[jimingham]

We claim in a couple places that the zeroth element of the module list for a target is the main executable, but we don't actually enforce that in the ModuleList class. As we saw, for instance, in

32dd5b20973bde1ef77fa3b84b9f85788a1a303a

it's not all that hard to get this to be off. This patch ensures that the first object file of type Executable added to it is moved to the front of the ModuleList. I also added a test for this.

In the normal course of operation, where the executable is added first, this only adds a check for whether the first element in the module list is an executable. If that's true, we just append as normal.

Note, the code in Target::GetExecutableModule doesn't actually agree that the zeroth element must be the executable, it instead returns the first Module of type Executable. But I can't tell whether that was a change in intention or just working around the bug that we don't always maintain this ordering. But given we've said this in scripting as well as internally, I think we shouldn't change our minds about this.

(cherry picked from commit a4cd99ea8736eda2b8b4de34453f55008bcf9c30)

[jimingham]

@swift-ci please test