[CIR] Handle throwing calls inside EH cleanup (#188341)
This implements handling for throwing calls inside an EH cleanup
handler. When such a call occurs, the CFG flattening pass replaces it
with a cir.try_call op that unwinds to a terminate block.
A new CIR operation, cir.eh.terminate, is added to facilitate this
handling, and the design document is updated to describe the new
behavior.
Assisted-by: Cursor / claude-4.6-opus-high
