Switch from C headers to C++ headers.

This CL makes the following substitutions.

* assert.h -> cassert
* math.h -> cmath
* stdarg.h -> cstdarg
* stdio.h -> cstdio
* stdlib.h -> cstdlib
* string.h -> cstring

stddef.h and stdint.h are not migrated to C++ headers.

PiperOrigin-RevId: 309074805

snappy_unittest.cc

@@ -26,8 +26,8 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-#include <math.h>
-#include <stdlib.h>
+#include <cmath>
+#include <cstdlib>
 
 #include <algorithm>
 #include <random>