Add Clear() function for RowKeyType to unblock LSC

Adds Clear(std::string& key) function that calls std::string::clear()
in OSS builds. Google-land will have corresponding overload for
absl::Cord::Clear() to replace deprecated absl::Cord::clear().

Signed-off-by: Dorna Raj Gyawali <dronarajgyawali@gmail.com>

Author: drona-gyawali

google/cloud/bigtable/internal/google_bytes_traits.h

@@ -38,6 +38,9 @@ namespace internal {
 /// Return true if the row key is empty.
 inline bool IsEmptyRowKey(std::string const& key) { return key.empty(); }
 
+/// Clear the row key.
+inline void Clear(std::string& key) { key.clear(); }
+
 /// Return true if the row key is empty.
 inline bool IsEmptyRowKey(char const* key) { return std::string{} == key; }