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-stubs-internal.h

@@ -35,15 +35,14 @@
 #include "config.h"
 #endif
 
-#include <cstdint>
+#include <stdint.h>
+
+#include <cassert>
+#include <cstdlib>
 #include <cstring>
 #include <limits>
 #include <string>
 
-#include <assert.h>
-#include <stdlib.h>
-#include <string.h>
-
 #ifdef HAVE_SYS_MMAN_H
 #include <sys/mman.h>
 #endif