[llvm] [opt-viewer] Allow users to set LLVM_HAVE_OPT_VIEWER_MODULES manually… (PR #151845)

[Ross Burton via llvm-commits]

rossburton wrote:

I know that LLVM currently has all the detection logic embedded in AddLLVM.cmake, is that by design or just how it's happened?  It feels like it could be moved into llvm/tools/opt-viewer/CMakeLists.txt which would then also automatically not run if LLVM_TOOL_OPT_VIEWER_BUILD was OFF.

https://github.com/llvm/llvm-project/pull/151845