[hwasan] work around lifetime issue with setjmp.

setjmp can return twice, but PostDominatorTree is unaware of this. as
such, it overestimates postdominance, leaving some cases (see attached
compiler-rt) where memory does not get untagged on return. this causes
false positives later in the program execution.

this is a crude workaround to unblock use-after-scope for now, in the
longer term PostDominatorTree should bemade aware of returns_twice
function, as this may cause problems elsewhere.

Reviewed By: eugenis

Differential Revision: https://reviews.llvm.org/D118647