[Sema] Implement support for -Wformat-signedness (#74440)

In gcc there exist a modifier option -Wformat-signedness that turns on
additional signedness warnings in the already existing -Wformat warning.

This patch implements that support in clang. This is done by adding a dummy
warning diag::warn_format_conversion_argument_type_mismatch_signedness that
is never emitted and only used as an option to toggle the signedness warning
in -Wformat. This will ensure gcc compatibility.