[llvm] [llvm] Fixes for compiling LLVM as a DLL (PR #210746)

[Saleem Abdulrasool via llvm-commits]

compnerd wrote:

I definitely don't understand the need for the complete type, the DLL annotation is all that matters - I could buy the argument that we need it to see the correct DLL storage annotation, but I'd say that we should fix the annotations on the forward declaration instead.

https://github.com/llvm/llvm-project/pull/210746