Protect exceptional paths under libcpp-no-exceptions

These tests are of the form

try {
   action-that-may-throw
   assert(!exceptional-condition)
   assert(some-other-facts)
 } catch (relevant-exception) {
   assert(exceptional-condition)
 }

Under libcpp-no-exceptions there is still value in verifying
some-other-facts while avoiding the exceptional case. So for these tests
just conditionally check some-other-facts if exceptional-condition is
false. When exception are supported make sure that a true
exceptional-condition throws an exception

Differential Revision: https://reviews.llvm.org/D26136

llvm-svn: 285697