Protect bitset tests under libcpp-no-exceptions

Bitset tests feature a sequence of tests of increasing bitset sizes,
but these tests rely on exceptions when the bitset size is less than
50 elements.

This change adds a flag to tell whether a test should throw. If it must
throw it will be skipped under no-exceptions.

Differential Revision: https://reviews.llvm.org/D26140

llvm-svn: 286474