[libc++] [modules] Defining __cpp_lib_modules

[ChuanqiXu9]

See cppalliance/boost2#1 (comment)

I guess the reason why libc++ doesn't define this is that it is still under experimental. I registered this issue to track this.

Maybe we can define the macro earlier to make it easier for people to try it?

[mordante]

Actual the real reason is that I have not verified whether the paper is completely implemented. I need to verify that.