Do not crash when trying to encode a _BitInt type

Using a _BitInt (or _ExtInt) type as a block parameter or block return
type hits an "unreachable" when trying to determine the encoding for
the block. Instead of crashing, this patch handles it like some of the
other types for which we don't yet have an encoding. The test case
verifies we no longer crash, but does not verify that we provide any
particular encoding (it can be updated once someone more familiar with
ObjC steps in to define the encoding).

Fixes PR50503.