[LTO] Avoid assert fail on failed pass plugin load (llvm#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.

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);
   }
 }

llvm/test/Feature/load_plugin_error.ll

@@ -1,5 +1,18 @@
-; REQUIRES: plugins, examples
+; REQUIRES: plugins
 ; UNSUPPORTED: target={{.*windows.*}}
 
-; RUN: not opt < %s -load-pass-plugin=%t/nonexistant.so -disable-output 2>&1 | FileCheck %s
-; CHECK: Could not load library {{.*}}nonexistant.so
+; RUN: not opt < %s -load-pass-plugin=%t/nonexistent.so -disable-output 2>&1 | FileCheck %s
+
+; RUN: opt %s -o %t.o
+; RUN: not llvm-lto2 run -load-pass-plugin=%t/nonexistent.so %t.o -o %t \
+; RUN:     -r %t.o,test 2>&1 | \
+; RUN:   FileCheck %s
+
+; CHECK: Could not load library {{.*}}nonexistent.so
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @test() {
+  ret void
+}