[C++20] [Modules] Make member functions with a in-class definition in HU implicitly inline

According to [dcl.inline]p7/note4,

> In the global module, a function defined within a class definition is
> implicitly inline.

And the declarations in the header unit are attached to the global
module fragment. So the function defined within a class definition in
header units should be implicitly inline too.

This fixes https://github.com/llvm/llvm-project/issues/57571.