Fix compilation errors under C++11.

`std::string::data()` is const-only until C++17.

PiperOrigin-RevId: 479708109

snappy.cc

@@ -2041,7 +2041,7 @@ size_t CompressFromIOVec(const struct iovec* iov, size_t iov_cnt,
                          std::string* compressed) {
   // Compute the number of bytes to be compressed.
   size_t uncompressed_length = 0;
-  for (int i = 0; i < iov_cnt; ++i) {
+  for (size_t i = 0; i < iov_cnt; ++i) {
     uncompressed_length += iov[i].iov_len;
   }
 

snappy_benchmark.cc

@@ -164,7 +164,7 @@ void BM_UIOVecSource(benchmark::State& state) {
   struct iovec iov[kNumEntries];
   size_t used_so_far = 0;
   for (int i = 0; i < kNumEntries; ++i) {
-    iov[i].iov_base = contents.data() + used_so_far;
+    iov[i].iov_base = const_cast<char*>(contents.data()) + used_so_far;
     if (used_so_far == contents.size()) {
       iov[i].iov_len = 0;
       continue;

snappy_unittest.cc

@@ -170,7 +170,7 @@ struct iovec* GetIOVec(const std::string& input, char*& buf, size_t& num) {
 int VerifyIOVecSource(const std::string& input) {
   std::string compressed;
   std::string copy = input;
-  char* buf = copy.data();
+  char* buf = const_cast<char*>(copy.data());
   size_t num = 0;
   struct iovec* iov = GetIOVec(input, buf, num);
   const size_t written = snappy::CompressFromIOVec(iov, num, &compressed);