Auto merge of #30233 - retep998:where-in-the-world-is-windows-sdk, r=…

…alexcrichton

What I've done here is try to make the code match what vcvars does much more closely. It now chooses which SDK to find based on the version of MSVC that it found. It also bases the decision of whether to find all the things on whether `VCINSTALLDIR` has been set, which is more likely to have only been set by an invocation of vcvars, unlike previously where it would do some things only if `LIB` wasn't set even though there was a valid use case for libraries to add themselves to `LIB` without having invoked vcvars.

There are still some debug `println!`s so people can test the PR and make sure it works correctly on various setups.

It supports VS 2015, 2013, and 2012. People who want to use versions of VS older (or newer) than that will have to manually invoke the appropriate vcvars bat file to set the proper environment variables themselves.

Do not merge yet.

Fixes #30229