if *none* of the successors of the call expression is the exit block.
This matters when a call of bool type is the condition of (say) a while
loop in a function with no statements after the loop.  This *can* happen
in C, but it's much more common in C++ because of overloaded operators.

Suppresses some substantial number of spurious -Wmissing-noreturn warnings.

llvm-svn: 102696