Introduce _LIBCPP_DEPRECATED_ABI_EXTERNAL_ERROR_CATEGORY_CONSTRUCTOR ABI option.

Currently libc++ compiles a special version of error_category()
into the dylib. This definition is no longer needed, and doesn't
work on Windows due to dllimport/dllexport semantics.

For those reasons this patch introduces an option to
disable/enable this definition. By default the definition
is provided in ABI v1 except on windows. This patch
also addresses D28210.

llvm-svn: 290840