Resolve "Error finding an LLVM build" (#1831)

[cmcneish]

Pass through Visual Studio version from build.sh into build/LLVM.lua, so that the downloaded version is consistent with build.sh's view rather than the host command prompt's view.

This should fix #1831.

Questions (1st time I've written any premake...):

• Does this only need to go to premakes with --file=LLVM.lua?
• Should the newoption exist? Premake didn't seem to need it.
• Should the newoption have a list of supported versions? Seems like build.sh is responsible for validation here, but I'm not sure

[cmcneish]

@dotnet-policy-service agree

[tritao]

The only problem I see with this is that now we will always set the vs option in Premake, so it will not act as an override.

I think you can rename the existing vs variable to action_vs and add a new vs that is empty by default. Then --vs=$vs will end up as --vs= and OPTIONS["vs"] will not be set anymore.

[cmcneish]

Sorry about the delay, I don't have too much time to work on this. (If you want to finish this up I don't mind)

I think you can rename the existing vs variable to action_vs and add a new vs that is empty by default. Then --vs=$vs will end up as --vs= and OPTIONS["vs"] will not be set anymore.

Is the idea here to provide a different override for LLVM's VS version than the one used to build CppSharp? Would this warrant a new top-level argument in build.sh (say, --llvm_vs)?