This commit adds a new initialization priority `FreezeNamespaces` that is run
last and moves all calls to `Namespace::Freeze()` there. This allows all other
initialization functions to still update namespaces without the use of the
`overrideFrozen` flag.
It also moves the initialization of `System.Platform*` and `System.Build*` to
an initialize function so that these can also be set without setting
`overrideFrozen`.
This is preparation for a following commit that will make the frozen flag in
namespaces finial, no longer allowing it to be overriden (freezing the
namespace will disable locking, so performing further updates would be unsafe).
Now that all values are in one place, there is no reason for this numbering
with gaps anymore. If you need to insert a new value in between, you can just
do so in the enum.
This reverses the sort order of the enum, thereby requiring a change to the
sort order of the std::priority_queue containing the elements.
Change the type of the priority values from int to a new enum. By replacing the
magic int values throughout the code base with named values, there is now a
single place where all priority values are defined and you get an overview over
the initialization order.
InitializeOnceHelper calls Loader::AddDeferredInitializer which takes a
std::function, so it's eventually converted to that anyways. This commit just
does this a bit earlier, and by saving the step of the intermediate C function
pointer, this would now also work for capturing lambdas (which there are none
of at the moment).
