With these changes, all these benchmarks
```cpp
BENCHMARK("Empty benchmark") {};
BENCHMARK("Throwing benchmark") {
    throw "just a plain literal, bleh";
};
BENCHMARK("Asserting benchmark") {
    REQUIRE(1 == 2);
};
BENCHMARK("FAIL'd benchmark") {
    FAIL("This benchmark only fails, nothing else");
};
```

report the respective failure and mark the outer `TEST_CASE` as
failed. Previously, the first two would not fail the `TEST_CASE`,
and the latter two would break xml reporter's formatting, because
`benchmarkFailed`, `benchmarkEnded` etc would not be be called
properly in failure cases.

This should also be done for test files, but that has lower priority.

This should improve the compilation times by decreasing the number
of TUs compiled, without making overly big TUs that would cause
problems with heavy-tailed compilation times.

There is one "combined TU" for the top level part, and each subpart,
except for Reporters, which currently do not have any trivial TUs.