Support invoking a benchmark by name.

Say I have:

```
BENCHMARK(Q1) {
// blah
}

BENCHMARK(Q2) {
// blah
}

BENCHMARK(Q3) {
// blah
}

int main() {
// In my main, I'd like pass arguments to control which benchmark to run

for (auto name: FLAG_benchmark_to_run) {
FOLLY_INVOKE_BENCHMARK_BY_NAME(name);
}
}
```

For example, how do I support:
```
# This skips Q2
./my_benchmark --benchmark_to_run=Q1,Q3
```

Is there anything like `FOLLY_INVOKE_BENCHMARK_BY_NAME` available?

The issue does not name any source files or tests. Start by locating the existing BENCHMARK registration and invocation entry points, then determine how named selection and comma-separated command-line values should work; done means Q1 and Q3 run while Q2 is skipped for the shown example.

Written by the indexing model from the issue text.