* Cleanup triggered by work on pragma for GCC -Wunused-but-set-parameter.

* Backing out changes to eigen.h (to be worked on later).

* Adding PYBIND11_WORKAROUND_INCORRECT_GCC_UNUSED_BUT_SET_PARAMETER in type_caster_base.h (apparently needed only for older GCCs).

* Apparently older compilers need a simpler overload for silence_unused_warnings().

* clang C++11 compatibility: removing constexpr

* Special case for MSVC 2017: `constexpr void` return

* Trying again without the silence_unused_warnings(const int *) overload.

* Separate macros for ALL_GCC, OLD_GCC_UNUSED_BUT_SET_PARAMETER

* Changing to __GNUC__ <= 2 (turning off)

* Refined condition for PYBIND11_WORKAROUND_INCORRECT_OLD_GCC_UNUSED_BUT_SET_PARAMETER.

* Quick experiment trying out suggestion by @henryiii

* Introducing macro: PYBIND11_INT_ARRAY_WORKING_AROUND_MSVC_CLANG_ISSUES

* Trying henryiii@ (void) expander idea.

* fix: apply simpler expression with fewer workarounds

* Purging new-but-already-obsoleted  macro, made possible by @henryiii's commit.

* Renaming `ALL_GCC` macro back to just `GCC` (because there is no `OLD` anymore, luckily).

* [actions skip] Adding "All GCC versions" to comment, to be clear about it.

Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>