Do not allocate a second "background" alloc id for the main allocation of a static.

Instead we re-use the static's alloc id within the interpreter for its initializer to refer to the `Allocation` that only exists within the interpreter.

compiler/rustc_const_eval/src/interpret/mod.rs

@@ -39,4 +39,5 @@ use self::{
 };
 
 pub(crate) use self::intrinsics::eval_nullary_intrinsic;
+pub(crate) use self::util::{create_static_alloc, take_static_root_alloc};
 use eval_context::{from_known_layout, mir_assign_valid_types};