Fixed LTO issue on no discard variable (#1761)

Improve `UseCharPointer()` (thus, `DoNotOptimize()`) under MSVC LTO,
make it actually escape the pointer and prevent it from being optimized away.

Author: tmiguelf

src/benchmark.cc

@@ -152,8 +152,16 @@ BENCHMARK_EXPORT std::map<std::string, std::string>*& GetGlobalContext() {
   return global_context;
 }
 
-// FIXME: wouldn't LTO mess this up?
-void UseCharPointer(char const volatile*) {}
+static void const volatile* volatile global_force_escape_pointer;
+
+// FIXME: Verify if LTO still messes this up?
+void UseCharPointer(char const volatile* const v) {
+  // We want to escape the pointer `v` so that the compiler can not eliminate
+  // computations that produced it. To do that, we escape the pointer by storing
+  // it into a volatile variable, since generally, volatile store, is not
+  // something the compiler is allowed to elide.
+  global_force_escape_pointer = reinterpret_cast<void const volatile*>(v);
+}
 
 }  // namespace internal