[C++20][Modules] Update handling of implicit inlines [P1779R3]

This provides updates to
[class.mfct]:
Pre C++20 [class.mfct]p2:
  A member function may be defined (8.4) in its class definition, in
  which case it is an inline member function (7.1.2)
Post C++20 [class.mfct]p1:
  If a member function is attached to the global module and is defined
  in its class definition, it is inline.

and
[class.friend]:
Pre-C++20 [class.friend]p5
  A function can be defined in a friend declaration of a
  class . . . . Such a function is implicitly inline.
Post C++20 [class.friend]p7
  Such a function is implicitly an inline function if it is attached
  to the global module.

We add the output of implicit-inline to the TextNodeDumper, and amend
a couple of existing tests to account for this, plus add tests for the
cases covered above.

Differential Revision: https://reviews.llvm.org/D129045