make project system to automatically refresh reference if missing dll shows up once solution is opened

_From @heejaechang on February 23, 2016 20:4_

Current project system doesn't track missing metadata dlls (I mean real metadata dll, not cross-language p2p dll). since it doesnt know where the dll is (msbuild doesnt give back resolved path), it won't register file change tracker nor let us know about the dll.

right now, we have 2 workarounds for it. one is doing rebuild inside of VS or clicking reference node in solution explorer.

both of action tells VS to refresh all references which will make VS to go thorough those missing dlls to see whether they exist now.

it would be nice if VS can detect situation where missing dlls are available now, and do reference refresh automatically rather than waiting for explicit user action such as "build" as long as we have some way to guess missing dlls location (since we dont have resolved path). ex, hint path, reference name in path format.

_Copied from original issue: dotnet/roslyn#9067_