Make sure upstream object files are added to staticlibs when compiling

with ThinLTO and cross-lang-lto.

Normally, when compiling with whole-crate-graph ThinLTO, we expect
rustc's LTO step to "uplift" upstream object files/LLVM modules to
the current set of compilation artifacts. Therefore the staticlib
creation code skips this uplifting. However, when compiling with
"cross-language LTO" (i.e. defer LTO to the actual linker), the LTO
step in rustc is not performed, so we have to take care of copying
upstream object files during archive creation (like we already do
when compiling without any LTO).

Author: michaelwoerister

src/librustc_codegen_llvm/back/link.rs

@@ -1626,8 +1626,12 @@ fn relevant_lib(sess: &Session, lib: &NativeLibrary) -> bool {
 fn is_full_lto_enabled(sess: &Session) -> bool {
     match sess.lto() {
         Lto::Yes |
-        Lto::Thin |
         Lto::Fat => true,
+        Lto::Thin => {
+            // If we defer LTO to the linker, we haven't run LTO ourselves, so
+            // any upstream object files have not been copied yet.
+            !sess.opts.debugging_opts.cross_lang_lto.enabled()
+        }
         Lto::No |
         Lto::ThinLocal => false,
     }