Support LLVMInstallDir and LLVMToolsVersion MSVC options

[checkraisefold]

What problem will this solve? Adds support for custom LLVM installation compiler options. This is helpful when using a version of LLVM that doesn't come with Visual Studio.

What might be a solution? Something like:

llvmdir "C:\\a\\b\\c"
llvmversion "15.0.0"

What other alternatives have you already considered? If this is not a significant enough option to be included, adding a patch locally.

Anything else we should know? https://docs.microsoft.com/en-us/cpp/build/clang-support-msbuild?view=msvc-170

[nickclark2016]

This should be a pretty easy first issue. If someone wants to tackle it, I'd be more than happy to review it. If not, I'll toss it on my todo backlog.

[GloriousPtr]

Any updates on this?

[NvSchobi]

I would be interested as well.

[nickclark2016]

Finally got to it, but I have a PR out to support this.