SE-0444: Fix interactions with Cxx interop.
With the upcoming `MemberImportVisibility` feature enabled, code built with Cxx
interop also enabled could be rejected by the compiler with cryptic errors
about the `__ObjC` module not being imported. This is the result of a
surprising implementation detail of Cxx interop. When importing C++ namespaces
and their members, the Clang importer puts these declarations in the Clang
header import module (a.k.a. the bridging header module, `__ObjC`). C++
namespaces don't have a logical modular home in the Swift AST because they can
span multiple modules, so it's understandable why this implementation was
chosen. However, the concrete members of namespaces also get placed in the
`__ObjC` module too, and this really confuses things.
To work around this idiosyncrasy of Cxx interop, I've introduced
`Decl::getModuleContextForNameLookup()` which returns the module that a
declaration would ideally belong to if Cxx interop didn't have this behavior.
This alternative to `Decl::getModuleContext()` is now used everywhere that
`MemberImportVisibility` rules are enforced to provide consistency.
Additionally, I found that I also had to further special-case the header import
module for Cxx interop because it turns out that there are some additional
declarations, beyond imported namespaces, that also live there and need to be
implicitly visible in every source file. The `__ObjC` module is not implicitly
imported in source files when Cxx interop is enabled, so these declarations are
not deemed visible under normal name lookup rules. When I tried to add an
implicit import of `__ObjC` when Cxx interop is enabled, it broke a bunch
tests. So for now, when a decl really belongs to the `__ObjC` module in Cxx
interop mode, we just always allow it to be referenced.
This Cxx interop behavior really needs a re-think in my opinion, but that will
require larger discussions.
Resolves rdar://136600598.
