[C++20] [Module] Support reachable definition initially/partially

This patch introduces a new kind of ModuleOwnershipKind as
ReachableWhenImported. This intended the status for reachable described
at: https://eel.is/c++draft/module.reach#3.

Note that this patch is not intended to support all semantics about
reachable semantics. For example, this patch didn't implement discarded
declarations in GMF. (https://eel.is/c++draft/module.global.frag#3).

This fixes: https://bugs.llvm.org/show_bug.cgi?id=52281 and
https://godbolt.org/z/81f3ocjfW.

Reviewed By: rsmith, iains

Differential Revision: https://reviews.llvm.org/D113545