If a module A exports a macro M, and a module B imports that macro and #undef's
it, importers of B should not see the macro. This is complicated by the fact
that A's macro could also be visible through a different path. The rules (as
hashed out on cfe-commits) are included as a documentation update in this
change.

With this, the number of regressions in libc++'s testsuite when modules are
enabled drops from 47 to 7. Those remaining 7 are also macro-related, and are
due to remaining bugs in this change (in particular, the handling of submodules
is imperfect).