This implements the fake return after a successful longjmp(), as
described in the linux man page:
Following a successful longjmp(), execution continues as if setjmp()
had returned for a second time. This "fake" return can be distin-
guished from a true setjmp() call because the "fake" return returns the
value provided in val. If the programmer mistakenly passes the value 0
in val, the "fake" return will instead return 1.
Reviewed By: sivachandra
Differential Revision: https://reviews.llvm.org/D146981
9b4e8b7b0a