Fix build errors regarding const qualifier being ignored on cast result type (#5432)

Summary:
This affects some TSAN builds:

env/env_test.cc: In member function ‘virtual void rocksdb::EnvPosixTestWithParam_MultiRead_Test::TestBody()’:
env/env_test.cc:1126:76: error: type qualifiers ignored on cast result type [-Werror=ignored-qualifiers]
       auto data = NewAligned(kSectorSize * 8, static_cast<const char>(i + 1));
                                                                            ^
env/env_test.cc:1154:77: error: type qualifiers ignored on cast result type [-Werror=ignored-qualifiers]
       auto buf = NewAligned(kSectorSize * 8, static_cast<const char>(i*2 + 1));
                                                                             ^
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5432

Differential Revision: D15727277

Pulled By: ltamasi

fbshipit-source-id: dc0e687b123e7c4d703ccc0c16b7167e07d1c9b0

env/env_test.cc

@@ -1123,7 +1123,7 @@ TEST_P(EnvPosixTestWithParam, MultiRead) {
 #endif
     ASSERT_OK(env_->NewWritableFile(fname, &wfile, soptions));
     for (size_t i = 0; i < kNumSectors; ++i) {
-      auto data = NewAligned(kSectorSize * 8, static_cast<const char>(i + 1));
+      auto data = NewAligned(kSectorSize * 8, static_cast<char>(i + 1));
       Slice slice(data.get(), kSectorSize);
       ASSERT_OK(wfile->Append(slice));
     }
@@ -1151,7 +1151,7 @@ TEST_P(EnvPosixTestWithParam, MultiRead) {
     ASSERT_OK(env_->NewRandomAccessFile(fname, &file, soptions));
     ASSERT_OK(file->MultiRead(reqs.data(), reqs.size()));
     for (size_t i = 0; i < reqs.size(); ++i) {
-      auto buf = NewAligned(kSectorSize * 8, static_cast<const char>(i*2 + 1));
+      auto buf = NewAligned(kSectorSize * 8, static_cast<char>(i*2 + 1));
       ASSERT_OK(reqs[i].status);
       ASSERT_EQ(memcmp(reqs[i].scratch, buf.get(), kSectorSize), 0);
     }