Generated map<string, string> code might trigger unused parameter warning

[georgthegreat]

What version of protobuf and what language are you using?
Version: master
Language: C++

What operating system (Linux, Windows, ...) and version? Linux, any will do

What runtime / compiler are you using (e.g., python version or gcc version)
clang or gcc, with -Wunused-parameter switch enabled

What did you do?
Steps to reproduce the behavior:

1. Use map<string, string> field in a message
2. Use syntax="proto2"; (default behavior)
3. Don't use optimize_for="lite_runtime"; (default behavior)
4. Get

error: unused parameter 'p' [-Werror,-Wunused-parameter]
      static void Check(ConstPtr p) {

in utf8 key / value validation.

Anything else we should know about your project / environment

The issue is triggered by incorrect check. At the time key / value validation method is invoked if either key or value is string.
However, the Utf8CheckMode must be taken into account and check generation / invokation should be omitted.

The fix will result in a number of pull requests. I am goind to attach them all to this issue.