Fix -Wunused-parameter in map<string, int> fields (fixes #8494)

[georgthegreat]

This is a clumsy fix for #8494.
MWE is as follows:

syntax = "proto2";
optimize_for = "lite_runtime";
message M {
   map<string, int> field = 1;
}

During code generation, protoc will check for key / value types (corresponds to string_key / string_value variables all around).
If Utf8 validation will be disabled due to evaluation of GetUtf8CheckMode() to kNone, the Check() method will be generated as follows:

static void Check(ConstPtr p) {
}

This will trigger -Wunused-parameter.

[georgthegreat]

@acozzette, could you, please, clarify the release timeline of protobuf=3.16 and protobuf=3.17 (as I see, 3.17.0-rc1 has just appeared in tags).

I have most of the patches submitted as PRs applied locally, and I am waiting for release to remove them. As I see, this PR is not included into 3.16.0-rc2 though it landed in the master some time ago.

Does protobuf follow the trunk based development practice?

[acozzette]

We ordinarily do what one would call trunk-based development, but this is just an unusual case where we need to do two releases in quick succession. 3.16 is going to be a long-term support release for Java and will get fixes for at least a year, but we have other changes we want to release in 3.17 which we don't want to commit to supporting for that long. 3.17 includes everything merged to master up to this morning and I expect to release it either by tomorrow or early next week.

[georgthegreat]

Thanks for the clarification!
I will wait for 3.17 to be released.