run-pass multidispatch tests trigger UAC on Windows

[klutzy]

UAC thinks it is something about patch :(
cc #10512 and #11207

[bombless]

Actually, rustc -o _dipatch or rustc -o dispatch_ will trigger UAC while rustc -o dispatch won't.

[ColonelJ]

I'm planning to figure out a way to fix #11207 and add proper support for manifests to Rust (I think all Windows binaries ought to have one, to enable visual styles, show application version number etc.), but as noted here as a temporary measure for this particular problem all you need to do is set an environment variable __COMPAT_LAYER as RunAsInvoker on your machine and you'll be able to run those tests fine.

I've made a pull request #21496 for this which make the test runner set this variable so you don't have to set it globally and affect the normal running of your box w.r.t. setup.exe and friends. Though I expect most Rust developers on Windows would be smart enough to figure out that a right click -> Run As Administrator might be required when it brings up some privilege related error! Even so, it's better if it just compiles and passes test 'out of the box' so to speak, after you've already gone through all the faff of installing some Rust-compatible MINGW (and Python, and whatever else...) on your machine.

[klutzy]

Fixed by #21496. (Executables still don't have manifest, but it's other issue: #10512/#11207)