Auto merge of #33169 - swgillespie:issue32829, r=eddyb

const_fn: Check the terminating expression of a block for blocks in a const initializer

In a const or static initializer, the `CheckBlock` check ensures that blocks in the initializer expression are only in tail positions or in items. In this case, it didn't check the terminating expression of a block, which resulted in an ICE later in the compiler pipeline if the trailing expression was itself a block. This change fixes the ICE and ensures that the proper error is emitted. This fixes the ICE in #32829 .