[C++20] [Modules] Don't complain about duplicated default template argument across modules

 See https://github.com/cplusplus/draft/pull/5204 for a detailed
 background.

 Simply, the test redundant-template-default-arg.cpp attached to this
 patch should be accepted instead of being complained about the
 redefinition.

 Reviewed By: urnathan, rsmith, ChuanqiXu

 Differential Revision: https://reviews.llvm.org/D118034