table/block_based_table_builder.cc: intentional fallthrough - comment to match gcc pattern

Summary:
The gcc-7 code for parsing comments (libcpp/lex.c) didn't match
the intentional fallthough in this comment.

table/block_based_table_builder.cc: In member function 'void rocksdb::BlockBasedTableBuilder::WriteRawBlock(const rocksdb::Slice&, rocksdb::CompressionType, rocksdb::BlockHandle*)':
table/block_based_table_builder.cc:754:22: error: this statement may fall through [-Werror=implicit-fallthrough=]
         assert(false);
                      ^
table/block_based_table_builder.cc:756:7: note: here
       case kCRC32c: {
       ^~~~
cc1plus: all warnings being treated as errors
Closes https://github.com/facebook/rocksdb/pull/1661

Differential Revision: D4318817

Pulled By: yiwu-arbug

fbshipit-source-id: e67d171

table/block_based_table_builder.cc

@@ -752,7 +752,7 @@ void BlockBasedTableBuilder::WriteRawBlock(const Slice& block_contents,
       case kNoChecksum:
         // we don't support no checksum yet
         assert(false);
-        // intentional fallthrough in release binary
+        // intentional fallthrough
       case kCRC32c: {
         auto crc = crc32c::Value(block_contents.data(), block_contents.size());
         crc = crc32c::Extend(crc, trailer, 1);  // Extend to cover block type