The Swift compiler uses an ugly hack that auto-imports a submodule's
top-level-module, even if we didn't ask for it. Reflect that in the
debug info.
<rdar://problem/31310320>
Currently -Xcc options are serialized in Swift modules, but they are
not saved as attributes to the DW_TAG_module representing the imported
clang module. This patch saves all *user-specified* -D macros there,
but it does not save any macros that are added by the ClangImporter
itself.
<rdar://problem/31990102>
