[lld/mac] Add a test for -reexport_library + -dead_strip_dylibs

Our behavior here already matched ld64, now we have a test for it.

(ld64 even strips the library here if you also pass -needed_library bar.dylib.
That seems wrong to me, and lld honors needed_library in that case.)

Differential Revision: https://reviews.llvm.org/D103812