lang/swift6: Unbreak build with clang 21

Clang 21 appears to have a regression in wchar.h handling when c++20
modules funcationality is enabled. (Swift compiler uses clang's
modules function under the hood.)

This is a workaround to avoid a compilation error by defining the
macro _WCHAR_H_CPLUSPLUS_98_CONFORMANCE_ to 1 before the compiler
reads the system wchar.h.

PR: 296000
Reported by: Tomoaki AOKI <junchoon@dec.sakura.ne.jp>
Tested by: Tomoaki AOKI <junchoon@dec.sakura.ne.jp>