[lld-macho] Bind re-exported symbols directly to implicitly-linked umbrellas

Suppose we are linking against libFoo, which re-exports the
implicitly-bound libSystem, which in turn re-exports some
non-explicitly-bound library like `/usr/lib/system/libsystem_c.dylib`.
Then any bindings we have to a symbol in libsystem_c should use
libSystem (and not libFoo) as the umbrella library.

Reviewed By: #lld-macho, smeenai

Differential Revision: https://reviews.llvm.org/D97865