Add optimizer barrier to benchmark calls of void-returning functions

The goal is to do the equivalent of clobbering memory, which forces
the compiler to keep side effects that are external to the benchmarked
function, while allowing it to optimize inside the benchmarked function.

E.g. this cannot be optimized away:
```cpp
BENCHMARK("foo") {
    global_count += 1;
};
```

while this can:
```cpp
BENCHMARK("bar") {
    size_t local_count = 0;
    for (size_t i = 0; i < 100; ++i) {
        local_count += 1;
    }
};
```

1 files changed, 0 insertions, 3 deletions

diff --git a/tests/SelfTest/IntrospectiveTests/InternalBenchmark.tests.cpp b/tests/SelfTest/IntrospectiveTests/InternalBenchmark.tests.cpp
index 69251d97..b7ec17b8 100644
--- a/tests/SelfTest/IntrospectiveTests/InternalBenchmark.tests.cpp
+++ b/tests/SelfTest/IntrospectiveTests/InternalBenchmark.tests.cpp
@@ -429,9 +429,6 @@ TEST_CASE("run benchmark", "[benchmark][approvals]") {
 }
 
 TEST_CASE("Failing benchmarks", "[!benchmark][.approvals]") {
-    SECTION("empty", "Benchmark that has been optimized away (because it is empty)") {
-        BENCHMARK("Empty benchmark") {};
-    }
     SECTION("throw", "Benchmark that throws an exception") {
         BENCHMARK("Throwing benchmark") {
             throw "just a plain literal, bleh";