Auto merge of #46750 - varkor:imp-llmod, r=estebank

Improve error messages on LLVM bitcode parsing failure

The LLVM error causing the parse failure is now printed, in the style
of the other thin LTO error messages. This prevents a flood of
assertion failure messages if the bitcode can’t be parsed.

src/librustc_trans/back/lto.rs

@@ -613,7 +613,10 @@ impl ThinModule {
             self.data().len(),
             self.shared.module_names[self.idx].as_ptr(),
         );
-        assert!(!llmod.is_null());
+        if llmod.is_null() {
+            let msg = format!("failed to parse bitcode for thin LTO module");
+            return Err(write::llvm_err(&diag_handler, msg));
+        }
         let mtrans = ModuleTranslation {
             source: ModuleSource::Translated(ModuleLlvm {
                 llmod,