[clang-tools-extra] [lld] [llvm] tools: Remove untested PluginLoader includes (PR #117644)

[Alexis Engelke via cfe-commits]

aengelke wrote:

- opt should be fine now? %loadPolly doesn't seem to exist anymore. Some tests might need to be updated to drop the legacy PM -load option.
- LTO (llvm-lto2+lld) would need changes to load PassPlugins before parsing -mllvm options, i.e. should be loaded immediately (like llc/opt do), which is annoying of course. Or we discourage plugins from registering cl opts and provide some other way to pass arguments to plugins (maybe similar to what clang does or something like `-load-pass-plugin=bye.so:-wave-goodbye`).
- clang-tidy and bugpoint seem to be the actual users of this mechanism.

https://github.com/llvm/llvm-project/pull/117644