[llvm] [llvm] get cl::opt instantiations working with MSVC DLL build (PR #147810)

[Andrew Rogers via llvm-commits]

andrurogerz wrote:

Per input from @compnerd, I am exploring eliminating `std::string` from the inheritance chain of `cl::opt<std::string>`, which would let us always export that template instance instead of special-case excluding it when building a DLL with MSVC. So let's hold off on merging this one.

https://github.com/llvm/llvm-project/pull/147810