[lld-macho] Support linking against stub dylibs

Stub dylibs differ from "real" dylibs in that they lack any content in
their sections. What they do have are export tries and symbol tables,
which means we can still link against them. I am unclear how to
properly create these stub dylibs; XCode 11.3's `lipo` is able to create
stub dylibs, but those lack LC_ID_DYLIB load commands and are considered
invalid by most tooling. Newer versions of `lipo` aren't able to create
stub dylibs at all.  However, recent SDKs in XCode still come with valid
stub dylibs, so it still seems worthwhile to support them. The YAML in
this diff's test was generated by taking a non-stub dylib and editing
the appropriate fields.

Reviewed By: #lld-macho, smeenai

Differential Revision: https://reviews.llvm.org/D89012