[llvm] [llvm] Guard CGTestPlugin folder under LLVM_ENABLE_PLUGINS uniformly (NFC) (PR #225362)

[Stefan Gränitz via llvm-commits]

weliveindetail wrote:

Thanks for the quick reply! I added two more small fixes:
* Move the lit test to Feature/codegen-plugin-legacy.ll, so it is in good company with codegen-plugin.ll and codegen-plugin-passes.mir
* Add CGTestPlugin to LLVM_TEST_DEPENDS

Not sure what went wrong with the Windows bot, but I had to rebase anyway. Let's see if it goes through now.

https://github.com/llvm/llvm-project/pull/225362