Cleanup of file-scoped and globally-scoped warning suppression pragmas across pybind11 header files. (#3201)
* Removing all MSVC C4127 warning suppression pragmas.
* Removing MSVC /WX (WERROR). To get a full list of all warnings.
* Inserting PYBIND11_SILENCE_MSVC_C4127. Changing one runtime if to #if.
* Changing PYBIND11_SILENCE_MSVC_C4127 macro to use absolute namespace (for use outside pybind11 include directory).
* Restoring MSVC /WX (WERROR).
* Removing globally-scoped suppression for clang -Wunsequenced. Based on an experiment under PR #3202 it is obsolete and can simply be removed.
