rustc: Don't deduplicate libraries linked to

Linker argument order with respect to libraries is important enough that we
shouldn't be attempting to filter out libraries getting passed through to the
linker. When linking with a native library that has multiple dependant native
libraries, it's useful to have control over the link argument order.
Expand nested items within a backtrace.

Fixes a regression from #4913 which causes items to be exanded with spans lacking expn_info from the context's current backtrace.
auto merge of #12688 : alexcrichton/rust/fix-some-link-args, r=brson

Linker argument order with respect to libraries is important enough that we
shouldn't be attempting to filter out libraries getting passed through to the
linker. When linking with a native library that has multiple dependant native
libraries, it's useful to have control over the link argument order.
auto merge of #12671 : nick29581/rust/expand, r=sfackler

Fixes a regression from #4913 which causes items to be exanded with spans lacking expn_info from the context's current backtrace.