This enables additional tests for the ClangImporter. This found a
missing piece in the `-enable-objc-interop` work that was done
previously. Address that and enable the tests. There are now the
following failing tests on Linux:
* sdk - depends on Foundation (not hermetic, see SR-7572)
* mixed-nsuinteger - depends on Foundation (not hermetic, see SR-7572)
* import-mixed-with-header-twice - requires apple/swift PR#16022
* can_import_objc_idempotent - requires apple/swift PR#16022
* objc_protocol_renaming - requires apple/swift PR#16022
If a bridging header imported a submodule but /not/ the top-level
module, the top-level module would never be made visible. This is a
problem since the Clang importer makes all top-level decls available
through their enclosing top-level modules, not the individual
submodules.
To fix this, stop special-casing the way we handle bridging header
imports; we can translate them from Clang Modules to Swift ModuleDecls
the same way we do for regular imports.
rdar://problem/37355259
