Fixed -Wunused-value warnings from Clang 3.4 in THROWS/NOTHROW use.

These warnings were emitted by test suites that included any test
cases for std::bad_cast exceptions being thrown or not thrown, as
per the following example:

    REQUIRE_THROWS_AS((bool)my_object, std::bad_cast);

See: https://github.com/philsquared/Catch/pull/262