This patch moves the following instances of this snippet:
auto components = builder.beginStruct();
CGVT.createVTableInitializer(components, VTLayout, RTTI);
into CGCXXABI under a virtual method called `SetVTableInitializer`, which will be in charge of selecting the correct vtable initialization from `CodegenVTables`.
This is a prerequisite for landing the relative vtables ABI in Fuchsia (see D72959). The plan is to add a `createRelativeVTableInitializer()` method to `CodegenVTables` which will be a helper for making PIC-friendly vtables.
This does not affect any tests.