[ThinLTO] Skip opt pipeline and summary wrapper pass on empty modules (#120143)

Follow up to PR118508, to avoid unnecessary compile time for an empty
combind regular LTO module if all modules end u

[ThinLTO] Skip opt pipeline and summary wrapper pass on empty modules (#120143)

Follow up to PR118508, to avoid unnecessary compile time for an empty
combind regular LTO module if all modules end up being ThinLTO only.

This required minor changes to a few tests to ensure they weren't empty.

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Fix test introduced by d29fdfbc4e3b

It breaks builds like:

https://lab.llvm.org/buildbot/#/builders/190/builds/744

[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
th

[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.

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[opt] Properly report errors when loading pass plugins (#69745)

All error messages here already contain the path to the plugin, so no
need to repeat it in opt.cpp.