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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Can we get rid of PluginLoader.h/cpp now as well? Also, precommit CI failures look relevant.

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