Make exception_wrapper::throwException() strict

Make exception_wrapper::throwException() strict

Summary:
The definition of `throwException()` is changed to add the `[[noreturn]]`
attribute for consistency with `std::rethrow_exception`. If the
`exception_wrapper` contains an exception, that is rethrown; if not, then
`std::terminate` is called.

An alternative design which threw an exception if the `exception_wrapper` did not contain an exception was rejected because of the ambiguity it introduced as to whether the exception thrown was wrapped or not.

Benchmarks before:

```

============================================================================
folly/test/ExceptionWrapperBenchmark.cpp        relative  time/iter  iters/s
============================================================================
exception_ptr_create_and_test                                1.45us  689.01K
exception_wrapper_create_and_test               4337.80%    33.46ns   29.89M
----------------------------------------------------------------------------
exception_ptr_create_and_test_concurrent                   342.99us    2.92K
exception_wrapper_create_and_test_concurrent     101.41%   338.21us    2.96K
----------------------------------------------------------------------------
exception_ptr_create_and_throw                               3.05us  327.89K
exception_wrapper_create_and_throw               140.22%     2.17us  459.77K
exception_wrapper_create_and_cast               8956.80%    34.05ns   29.37M
----------------------------------------------------------------------------
exception_ptr_create_and_throw_concurrent                  372.68us    2.68K
exception_wrapper_create_and_throw_concurrent    102.54%   363.44us    2.75K
exception_wrapper_create_and_cast_concurrent     110.93%   335.97us    2.98K
============================================================================
```

and after:

```

============================================================================
folly/test/ExceptionWrapperBenchmark.cpp        relative  time/iter  iters/s
============================================================================
exception_ptr_create_and_test                                1.46us  684.09K
exception_wrapper_create_and_test               4368.84%    33.46ns   29.89M
----------------------------------------------------------------------------
exception_ptr_create_and_test_concurrent                   341.20us    2.93K
exception_wrapper_create_and_test_concurrent      99.88%   341.60us    2.93K
----------------------------------------------------------------------------
exception_ptr_create_and_throw                               3.08us  324.93K
exception_wrapper_create_and_throw               106.93%     2.88us  347.46K
exception_wrapper_create_and_cast               9532.05%    32.29ns   30.97M
----------------------------------------------------------------------------
exception_ptr_create_and_throw_concurrent                  363.73us    2.75K
exception_wrapper_create_and_throw_concurrent    101.04%   360.00us    2.78K
exception_wrapper_create_and_cast_concurrent     104.50%   348.07us    2.87K
============================================================================
```

Reviewed By: yfeldblum

Differential Revision: D3923939

fbshipit-source-id: 6fc58ac571e59e4122f1fbd194c678e3a3841057