[LTO] Avoid assert fail on failed pass plugin load (#96691)

Without this patch, passing -load-pass-plugin=nonexistent.so to
llvm-lto2 produces a backtrace because LTOBackend.cpp does not handle
the error correctly:

```
Failed to load passes from 'nonexistant.so'. Request ignored.
Expected<T> must be checked before access or destruction.
Unchecked Expected<T> contained error:
Could not load library 'nonexistant.so': nonexistant.so: cannot open shared object file: No such file or directoryPLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
```

Any tool using `lto::Config::PassPlugins` should suffer similarly.

Based on the message "Request ignored" and the continue statement, the
intention was apparently to continue on failure to load a plugin.
However, no one appears to rely on that behavior now given that it
crashes instead, and terminating is consistent with opt.

1 files changed, 2 insertions, 6 deletions

diff --git a/llvm/lib/LTO/LTOBackend.cpp b/llvm/lib/LTO/LTOBackend.cpp
index 84a69d9..d5d642f 100644
--- a/llvm/lib/LTO/LTOBackend.cpp
+++ b/llvm/lib/LTO/LTOBackend.cpp
@@ -191,12 +191,8 @@ static void RegisterPassPlugins(ArrayRef<std::string> PassPlugins,
   // Load requested pass plugins and let them register pass builder callbacks
   for (auto &PluginFN : PassPlugins) {
     auto PassPlugin = PassPlugin::Load(PluginFN);
-    if (!PassPlugin) {
-      errs() << "Failed to load passes from '" << PluginFN
-             << "'. Request ignored.\n";
-      continue;
-    }
-
+    if (!PassPlugin)
+      report_fatal_error(PassPlugin.takeError(), /*gen_crash_diag=*/false);
     PassPlugin->registerPassBuilderCallbacks(PB);
   }
 }