[Driver] Fix Windows SDK Detection

This fixes a couple of bugs in Windows SDK Detection.

1. `readFullStringValue` returns a bool, but was being compared
   with ERROR_SUCCESS.
2. `RegQueryValueExW` might return the null terminator in the
   queried value which will result in incorrect values being
   returned from `getSystemRegistryString`.

Patch By: comicfans44@gmail.com
Reviewed By: zturner
Differential Revision: http://reviews.llvm.org/D21946

llvm-svn: 277005