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

[Ross Burton via llvm-commits]

rossburton wrote:

Note that the opt-viewer script is always installed anyway, even if the dependencies are not present. I'd say the same variable should be used to seed whether opt-viewer is installed or not.

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