Locate VS InstallDir in the presence of newer runtime

This fixes getSystemRegistryString() in WindowsToolChain.cpp to
make sure that the VS version that it picks has an InstallDir.
Previously we would look for the highest version os VS and check
for InstallDir afterwards.

Patch by Yaron Keren!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192374 91177308-0d34-0410-b5e6-96231b3b80d8